Improving a Laptop Keyboard with Custom Keycap Veneers

I admit that I'm picky about the keyboards I use.  I'm accustomed to the general ergonomics of classic keyboards and have become spoiled by a Model M for the last 20 years.  While I can hardly expect to get a comparable experience from a laptop keyboard, I do feel that some desirable aspects of design are painstakingly avoided for the sake of petty fashion.  The keyboard on my laptop has been one of the most frustrating I have ever attempted to use.  At some point, I decided that there must be something I could do to turn it into a more comfortable and efficient keyboard.

Since full-size keyboards are where I find the most comfort, perhaps it's worth describing what aspects of their design I feel are important to address in my efforts.  While that discussion is likely going to suggest a mechanical keyboard as a central ideal, I don't think that any particular mechanism is as important as what properties it can impart to the product.  Bear in mind that much of this is merely my preference.

An old XT keyboard, showing cylindrical crowns and overall contour.

The most commonly noted aspects in which laptop and typical desktop keyboards differ have to do with the key stroke.  The mechanical characteristics of the key stroke are primarily a comfort concern, though they may also provide forms of feedback which promote speed -- or at least confidence in motion at speed. Hysteretic mechanisms can provide tactile feedback, higher actuation force makes the motions assertive, and long stroke length allows for natural follow-through.  While users of discrete mechanical keyboards may have a broad range of stroke characteristics to explore, it's reasonable to expect that any practical laptop keyboard is going to necessarily have a restricted range of possible characteristics.

While an unfamiliar laptop key layout may certainly conflict with motor memory, even a familiar layout becomes difficult to use without some form of orienting information.  It is the physical geometry of the keyboard which provides spatial cues to reinforce and maintain the accuracy of learned motions when moving from key to key.  Consider a similar sensorimotor task; it's easy to walk through a familiar room in darkness if you can occasionally touch known surfaces along the way.  Doing the same without any references is an exercise in error accumulation.  In terms of a continuous process, these forms of feedback facilitate the error determination necessary for error correction to be possible; they allow the control loop to be closed. 

The most obvious and universal orienting features are identifier bumps.  They are usually only located on the F and J keys, as well as the numpad 5 key (on keyboards with a numpad).  Bumps are one of the few physical features of keycaps that people can easily customize.  While self-adhesive bumps can be bought for this purpose, other methods such as glue are common.  Custom bumps are especially useful in emphasizing keys associated with certain keyboard commands.

While the typical keyboard bumps are a mechanism to orient hand position within a general area of the keyboard, it is the feedback provided by the shape of the keycaps and the overall contour of the keyboard itself which reinforces the discreteness of all keys.  The center of each key can be emphasized by making the crown of the keycap slightly concave, or by making the crown smaller than the pitch distance between key centers.  Providing strong centering cues helps maintain spatial awareness and helps to reduce the tendency for edge strikes and two-key strikes.  Many keyboards have the rows laid out in either a curved or linear stairstep fashion, a feature which both adds to the distinctness of the rows and aids in comfortably reaching the upper rows.

You could at least pretend that ergonomics matter.

The keyboard on this old laptop (Acer 7736z) had none of that going for it. Being a laptop, the keyboard is flat (planar); that much is a necessity, though it isn't exactly helping.  The level of the keyboard is actually slightly lower than the body of the laptop, which makes the hand positions uncomfortably low, turning the overly-sensitve trackpad into an even worse nuisance.  The keystroke is light, short, and ambiguous.  Every keystroke is like the experience of stomping at the top of a long flight of stairs when you thought there was still one more step to go. The keycaps are perfectly flat, with no crown and no functional centering cues.  The key identifiers (bumps) are tiny and barely noticeable with dry hands.  Just trying to find home row after using the mouse is a tedious routine of sliding fingers around in a broad expanse of flat slippery plastic, all the while trying to maintain the lightest of touch to avoid errant keystrokes.  Once my hands move away from a known position, the lack of any discernable spatial references is immediately disorienting.  Combine that with the lack of feedback, and the whole experience becomes both tedious and precarious, like trying to touch-type with chopsticks.  Keyboard commands and programming both involve broader reaching motions than writing in simple prose, and are the most tiring of all.  The only way a keyboard could be ergonomically worse is if it were entirely featureless and truly devoid of feedback, like a touchscreen OSK or projection keyboard. 

In all the time I've spent using this laptop, nearly every minute has been steeped in the thought that there must be a way to improve its keyboard.  As per the mentioned limits of practicality, there is little I can do to alter the keystroke or overall contour.  While I can't alter the layout, perhaps I can add some extra identifiers.  Adding various forms of spatial cues to the keycaps should be possible, though there are some limitations.  Of course, there's the limitation of the distance between the keyboard and screen when the laptop is closed.  It might also be desirable to make sure any alterations are removable; after all, this is likely to require some experimentation. 

The First Attempt at a Solution

My first thought was to use adhesive to add identifier bumps to certain keys.  After some thought, I came to the prior conclusions regarding more general spatial awareness reinforcement, ultimately deciding that a centering mark of some sort should be added to the most-used keys.  My thought was to simulate the effect of a concave keycap crown by adding a circular ridge on each key.  I could add ancillary bumps or ridges either for location, identification, or for avoidance. 

While a finalized design could be implemented using epoxy, I opted to prototype my ideas using a conformal coating made from clear Dap Sidewinder thinned with xylene.  This can be thinned to an appropriate consistency and provides a smooth, self-leveling finish.  It can also be completely removed once dry by simply redissolving in xylene; though in this case, sound material could be picked/peeled off cleanly without solvent.  It may also be possible to use something like E6000 if thinned appropriately. 

I applied the coating with a 1mL syringe and a 25ga dispensing needle bent to a comfortable angle.  There are a couple of important things that need to be considered in that effort.  Dispensing a viscous material through a fine needle requires a lot of pressure.  The pressure requirements can be lowered by reducing the viscosity or by using a shorter needle or one of larger diameter.  I chose a 1mL syringe because it allowed me to generate relatively high pressures, but care must be taken to avoid making a mess on the keyboard.  With firm hand strength, a 1mL syringe can produce upwards of 200 PSI -- more than enough to eject the needle and blurt a wad of glue across your work.  If you can use a syringe with a luer-lok type spigot, do so.  If all you can get are syringes with plain tapered spigots, pay attention to keep the taper clean and tight.  Practice first.

One also should consider the materials used.  Many adhesives and coatings will degrade when in constant contact with skin oils.  While I chose this coating for prototyping, it is wholly unsuited for long-term use.  After a week or so, it will begin to become sticky and will eventually get smeared everywhere.  Other products like E6000 will likely do the same on a longer timeframe, and I would expect the same of almost every common adhesive other than epoxy.

While I found it helpful to have more noticeable identifier bumps, the circular ridges left me relatively disappointed.  The rings were certainly better than nothing, but they were still a poor simulant of a key crown.  The fact that the keycap is the same height inside and outside the ring makes them fairly ineffective at suggesting a boundary.  With this effect in mind and considering the low height of the glue ridge, the ring diameter needed to be fairly small to present an unambiguous shape to the fingertip.  The smaller the rings are, the more awkward they feel, and the more often they're escaped. 

Placing my hands in the rings on home row felt awkward -- as if they're too straight.  Certainly, the wear marks on my main keyboard suggest that my fingertips naturally rest in an arc across the keys.  It followed that the landing points were generally centered laterally, but otherwise varied depending on what was most comfortable for a given finger.  I switched the circular glue rings out for elongated rectangular glue rings and noted a marginal improvement.  I felt that it was ultimately impractical to achieve much more with such a low-profile method. 

The Development of Keycap Veneers

Finding myself idle on my laptop away from home, I dared to spend some time on what I'd presumed would surely be a huge waste of time.  I figured I'd whip up a parametric model for 3D-printed keycaps.  The idea was to print some caps (if they would print flat and thin), clean them up (easier said than done), and glue them like veneers on top of the existing keys.  The modelling was done in OpenSCAD, a simple and enjoyable script-based parametric 3D CAD tool.  I conjured up several different variations on crown geometry, printing and testing along the way. 

// //////////////////////////////////////////////////////
// keycap veneers for shitty flat laptop keyboards

// cylinder cuts only strongly enforce x-positioning
// this allows fingertips to rest in a more natural arc across home row
// and may be more comfortable on keys which require more reaching or are otherwise habitually struck off-center
// they're also easier to make smooth (easier to print without fuzz, also easier to scrape/sand by hand)
// spherical cuts reinforce y-pos more than cylinder cuts do (important on flat kb)
// elliptical cuts are a compromise especially suited to flat profile kb

// proportions aren't fixed. some things require manual adjusting

// //////////////////////////////////////////////////////
// RENDERING & MULTIPART LAYOUT
nf=100;   // facet number (~50 for speed; ~200 for printing)
Nx=4;    // number of caps to tile along X-axis
Ny=2;    // number of caps to tile along Y-axis
tilegap=2;  // gap between tiles

// //////////////////////////////////////////////////////
// BASIC KEYCAP GEOMETRY
h=17;      // keycap height (in key plane)
w=17;    // keycap width (in key plane)
th=1.7;    // maximum cap thickness (limited by kb-screen gap)
th_min=0.3;  // minimum cap thickness (limited by print strength)
draftx=45;  // draft angle (vertical taper on R,L faces)
drafty=45;  // draft angle (vertical taper on T,B faces)
cr=3;     // corner radius 

// //////////////////////////////////////////////////////
// PRIMARY RELIEF CUTS
// sphere & cylinder mimic legacy alpha key designs 
// lcylinder is the transverse version of 'cylinder', for wide keys
// wcylinder is a double-cylindrical hull, for wide keys
// sausage is a double-spherical hull, for wide/tall keys
// ysausage is the same as 'sausage', but each sphere location can be independently offset
// bumps is a series of spherical bumps (used as an avoidance indicator) 
// using 'none' will produce a flat keycap
style="sphere";

// these parameters may need tweaked when geometry is changed significantly
drc=20;     // relief cut radius for cylinder styles
drs=20;     // relief cut radius for sphere & sausage styles
// the following options only apply to spherical cuts
osx=0;     // offset x
osy=0;     // offset y
osz=0;     // offset z
scaley=1.3;   // stretch factor (elliptical cut)
// the following options only apply to ysausage cuts
os1=-.5;     // y-offset for top sphere
os2=-3;     // y-offset for bottom sphere
// the following options only apply to 'bumps'
brad=1;     // bump radius
bdepth=0.7;   // bump depth
blayout=[3,4];  // number of bumps [x,y]

// //////////////////////////////////////////////////////
// ADDITIONAL FEATURES
// bevel produces a single beveled edge (e.g. for bottom-row keys)
// multiple bevels can be specified (e.g. ["top","bottom"])
bevel="none";   // bottom top left right or none
bevangle=10;  // angle of beveled face
bevhos=0.6;   // height offset of bottom edge of bevel

// cuts can be made asymmetric so that a single cut spans multiple keys
// this allows certain keys to be grouped by touch (e.g. groups of four F-keys)
lowside="none";  // right left both or none

// identifier position may be in the center or bottom edge
identifier="none";  // center edge or none
idw=0.5;   // identifier width
idl=5;     // identifier length
idh=0.95;   // identifier height WRT cap height (used for 'edge')
idhc=0.5;   // height used for "center"


// //////////////////////////////////////////////////////
// //////////////////////////////////////////////////////
// THE MAGIC

module cap(){
 color("dimgray")
 render(){
  linear_extrude(height=th,scale=[1-2*th*tan(drafty)/w,1-2*th*tan(drafty)/h]){
   hull(){
    translate([(w/2-cr),(h/2-cr),0])
     circle(r=cr,center=true,$fn=nf/5);
    translate([-(w/2-cr),-(h/2-cr),0])
     circle(r=cr,center=true,$fn=nf/5);
    translate([(w/2-cr),-(h/2-cr),0])
     circle(r=cr,center=true,$fn=nf/5);
    translate([-(w/2-cr),(h/2-cr),0])
     circle(r=cr,center=true,$fn=nf/5);
   }
  }
    }
}

module relief_bumps(){
 render(){
   difference(){
    translate([-0.6*w,-0.6*h,th-bdepth])
     cube(1.2*[w,h,brad]);
    union(){
     for (m=[1:blayout[1]]){
      for (n=[1:blayout[0]]){
       translate([-(w-w/blayout[0])/2+w/blayout[0]*(n-1),-(h-h/blayout[1])/2+h/blayout[1]*(m-1),th-brad])
        sphere(r=brad,center=true,$fn=nf/5); 
      }
     }
    }
   }
 }
}

module relief_spherical(){
 render(){
  translate([osx,osy,drs+th_min+osz]){
   sphere(r=drs,center=true,$fn=nf);
   if (lowside=="right" || lowside=="both")
    translate([h/2+osx,osy,0])
     rotate([90,0,90])
      cylinder(r=drs,h=h,center=true,$fn=nf);
   if (lowside=="left" || lowside=="both")
    translate([-h/2+osx,osy,0])
     rotate([90,0,90])
      cylinder(r=drs,h=h,center=true,$fn=nf);
  }
 }
}

module relief_cylindrical(){
 render(){
  translate([0,0,drc+th_min]){
   rotate([90,0,0])
    cylinder(r=drc,h=h+1,center=true,$fn=nf);
   if (lowside=="right" || lowside=="both")
    translate([h/2,0,0])
     rotate([90,0,90])
      cube([drc*2,drc*2,h],center=true);
   if (lowside=="left" || lowside=="both")
    translate([-h/2,0,0])
     rotate([90,0,90])
      cube([drc*2,drc*2,h],center=true);
  }
 }
}

module relief_cylindrical_long(){
 render(){
  translate([0,0,drc+th_min]){
   rotate([90,0,90])
    cylinder(r=drc,h=w+1,center=true,$fn=nf);
  }
 }
}

module relief_cylindrical_wide(){
 render(){
  translate([0,0,drc+th_min]){
   rotate([90,0,0])
    hull(){
     translate([(w-h)/2,0,0])
      cylinder(r=drc,h=h+1,center=true,$fn=nf);
     translate([-(w-h)/2,0,0])
      cylinder(r=drc,h=h+1,center=true,$fn=nf);
    }
   if (lowside=="right" || lowside=="both")
    translate([h/2,0,0])
     rotate([90,0,90])
      cube([drc*2,drc*2,h],center=true);
   if (lowside=="left" || lowside=="both")
    translate([-h/2,0,0])
     rotate([90,0,90])
      cube([drc*2,drc*2,h],center=true);
  }
 }
}

module relief_sausage(){
 render(){
  translate([0,0,drs+th_min]){
   hull(){
    translate([(w-h)/2,0,0])
     sphere(r=drs,center=true,$fn=nf);
    translate([-(w-h)/2,0,0])
     sphere(r=drs,center=true,$fn=nf);
   }
   if (lowside=="right" || lowside=="both")
    translate([h/2,0,0])
     rotate([90,0,90])
      cylinder(r=drs,h=h,center=true,$fn=nf);
   if (lowside=="left" || lowside=="both")
    translate([-h/2,0,0])
     rotate([90,0,90])
      cylinder(r=drs,h=h,center=true,$fn=nf);
  }
 }
}

module relief_ysausage(){
 render(){
  translate([0,0,drs+th_min]){
   hull(){
    translate([0,os1,0])
     sphere(r=drs,center=true,$fn=nf);
    translate([0,os2,0])
     sphere(r=drs,center=true,$fn=nf);
   }
   if (lowside=="right" || lowside=="both")
    translate([0,w/2,0])
     rotate([90,0,0])
      cylinder(r=drs,h=h,center=true,$fn=nf);
   if (lowside=="left" || lowside=="both")
    translate([0,-w/2,0])
     rotate([90,0,0])
      cylinder(r=drs,h=h,center=true,$fn=nf);
  }
 }
}

module relief_bevel(){
 union(){
  for (i=bevel){
   if (i=="top")
    translate([0,h/2,th_min+bevhos])
     rotate([bevangle,0,180])
     translate([0,h/4,w/2])
     rotate([90,0,0])
      cube([w,w,h/2],center=true);
   if (i=="bottom")
    translate([0,-h/2,th_min+bevhos])
     rotate([bevangle,0,0])
     translate([0,h/4,w/2])
     rotate([90,0,0])
      cube([w,w,h/2],center=true);
   if (i=="right")
    translate([w/2,0,th_min+bevhos])
     rotate([bevangle,0,90])
     translate([0,w/4,h/2])
     rotate([90,0,0])
      cube([h,h,w/2],center=true);
   if (i=="left")
    translate([-w/2,0,th_min+bevhos])
     rotate([bevangle,0,-90])
     translate([0,w/4,h/2])
     rotate([90,0,0])
      cube([h,h,w/2],center=true);
  }
 }
}

module drawcap(){
 difference(){
  cap();
  
  if (style=="sphere")
   if (scaley!=1)
    scale([1,scaley,1])
     relief_spherical();
   else
    relief_spherical();
  if (style=="cylinder")
   relief_cylindrical();
  if (style=="lcylinder")
   relief_cylindrical_long();
  if (style=="wcylinder")
   relief_cylindrical_wide();
  if (style=="sausage")
   relief_sausage();
  if (style=="ysausage")
   relief_ysausage();
  if (style=="bumps")
   relief_bumps();
  if (bevel!="none")
   relief_bevel();
 }
 
 if (identifier=="edge")
  translate([-idl/2,-idw/2-h*0.34,th_min])
   cube([idl,idw,th*idh-th_min+0.001]);
 else if (identifier=="center")
  translate([-idl/2,-idw/2,th_min])
   cube([idl,idw,th*idhc-th_min+0.001]);
}

module tilecaps(){
 color("dimgray"){
  for (m=[1:Ny]){
   translate([0,(m-1)*(h+tilegap),0]){
    for (n=[1:Nx]){
     translate([(n-1)*(w+tilegap),0,0]){
      drawcap();
     }
    }
   }
  }
 }
}

tilecaps();

Much of the core focus of these experiments stems from the lessons of the glue ring experiment; that is, finding the balance between the axial components of the crown geometry.  The equivalent analog for the circular glue ring is of course a spherically concave crown.  By contrast, many keyboards such as the Model M have cylindrically concave crowns.  While the spherical crowns provide constraint cues both laterally (side to side) and transversely (across the rows), cylindrical crowns only provide strong constraint laterally.  Much like using elongated glue rings, this allows for more variation of finger placement in the transverse direction without affecting comfort.  While the cylindrical shapes are certainly easier to deburr and finish than spherical ones, the overall flatness of the keyboard and the tendency to use the laptop in awkward positions without a desk left me wanting more transverse constraint than they could provide.  After all, while the Model M uses cylindrical crowns, it also has a significant and unambiguous height difference between rows.  The solution was simply a deep ellipsoidal crown contour.  Using atypically deep crowns helps compensate for the flatness inherent to a laptop keyboard, and generally provides more constraint overall.  A few quick tweaks were required to find what I felt was a good balance.  Compared to either spherical or cylindrical reliefs of a similar depth, the final ellipsoidal crown allowed a dramatic improvement in speed, accuracy, and comfort within the alpha keys.

Caps with spherical, cylindrical, and ellipsoidal crowns (and an identifier bump)

Wary of losing track of home row (something that was easy to do on the original keyboard), I decided to use different caps for the numeric row.  In order to provide a sense of overall contour, I opted to reduce the degree to which these cap crowns were contoured themselves.  In other words, any concave relief should be shallower, allowing the cap to be effectively thicker in the strike location.  Again, I used wear marks on a well-worn keyboard to locate the crown features.

The F-keys are grouped into blocks of four, with each group sharing an elongated concave relief.  This addresses the fact that the keys are not spaced or located as they are on a standard keyboard.  Only ESC and DEL have their own spherical relief. 

Keys which I rarely use, or keys which are otherwise hit accidentally need some sort of avoidance identifier.  I opted for a lower-profile cap with a grid of bumps.  While it makes for frustrating print cleanup, it's an effective solution. 

Other keys were given either shallow concave reliefs or simple bevels.  In this way, they are brought up to a comparable height with the other modified keys, even if they otherwise do not require any particular improvement.

Other cap types for function-row and numeric-row keys

The caps were printed in PLA, though this makes them difficult to clean up and finish.  The surfaces require quite a bit of burr removal or "defuzzing".  Most of this can be done by scraping, though the shapes of the parts often makes it awkward and the process is generally tedious.  Sanding is likewise very tedious, especially corners and edges.  Cylindrical-relief keys can be printed in rows and sanded in a comfortable motion, though spherical or ellipsoidal reliefs don't allow this.  Sanding tends to leave more fine fuzz.

Improving the finish beyond that is difficult.  As the parts are very thin, heat polishing (flame or hot air) doesn't really work.  The features are either destroyed, or the part curls and requires restraint -- leading to more marring.  Solvent polishing PLA is not really a thing.  No, acetone, MEK, and ethyl acetate don't work.  They might slightly soften and deglaze the part, but they will not dissolve the surface enough to allow it to reflow.  I do not know of any other solvents that would, but if there were one, I have a feeling that the parts would be so thin that they would tend to curl anyway.

Sanded, fuzzy caps temporarily affixed during fitting

Using a rotary tool to buff or burnish the surface is fairly counterproductive.  The tool needs to run much slower than they can, otherwise the heat generated by friction melts and smears the material due to its very low melting point.  Wool buffs, brushes, cratex points, and scotch brite bobs all ended up just making a mess.  In practice, the first thing they do is erase edges and corners. 

I ended up opting to just coat the caps, though I know that coatings on a keyboard are likely going to eventually fail.  I figured that my best bet was either polyurethane or a good clear acrylic.  I didn't feel like applying the poly with a brush or sprayer, so I just used an aerosol acrylic spray. No, I did not just spray the keys in-situ.  I used double-sided tape to hold them down to a waste board when spraying. 

The keycaps after a sloppy spray job. It feels better than it looks.

The caps were originally prototyped using simple craft rubber cement for adhesion.  This allowed them to easily be removed with a knife, and the residual glue simply rolled off the surfaces cleanly.  For final reassembly, I used a more proper polychloroprene contact cement (e.g. Weldwood, Barge).  Removal may now be more difficult, but possible.  Instead of brushing the cement on, I used a syringe to dispense a small amount on each key.  There are plenty of other adhesives that could be used here.  I figured that replacement keyboards are cheap enough that I don't really need to worry about removing the caps once I've settled on the design. 

Final Thoughts

The keyboard as modified has proven to be about as good as a laptop keyboard can possibly be.  The height is much more comfortable, the alpha keys are distinct from surrounding keys.  The identifier bumps are distinct, and I added a couple extra where I wanted them.  The finish isn't very pretty, but beauty isn't necessary here. 

If you read all that blather without losing interest or the will to live, I should congratulate you.  Even I had a hard time enduring it. 

Addressing the Minimum COL Limit for the Marlin 1894 Action

As is typical of the remainder of this blog, I am writing this following an experience with a problem about which I found frustratingly little clear information online.  That is not to say that I found little information; rather, I found a great wealth of conflicting, inaccurate, or at least confusing information obfuscating simple, but perhaps nonobvious truths. 

Let's start where the experience starts -- a rifle owner buys some new boxes of 44 Special (240gr LSWC) ammunition to use in his ca. 1971 Marlin 1894 chambered for 44 magnum.  While the first few rounds worked, the rims eventually start jamming between the lifter and the mouth of the magazine tube.  This was unexpected; all the prior boxes of old 246gr LRN 44 Special ammunition worked fine, and still do.  Why is the rifle jamming with the new ammunition?  My eyes see an obvious suspect, but let's get a second opinion.

HSM 44SPL (240gr LSWC), Winchester 44SPL (246gr LRN), Winchester 44MAG (240gr JSP)

If one immediately googles a relevant query, forums across the web will authoritatively dispense the answer with certainty. It's the "dreaded Marlin Jam", which is variously described as:

• not a thing
• very much a thing
• being caused by nothing other than short-stroking or holding the phone rifle wrong
• being caused by nothing other than extreme wear or abuse
• being caused by certain ammunition/gun combinations
• all Marlin's fault and they should have fixed this a hundred years ago

There are grains of truth spread nonuniformly throughout those discussions, but there are also a lot of misunderstandings and conflations going on as well.  It's easy to see how that happens.  The 1894 action is definitely susceptible to feeding problems, whether it is a consequence of the lack of controlled feed or the weaknesses of the simple mechanical logic going on inside.  It's also frustratingly difficult to visualize what exactly is going on inside.  There isn't a good way to see or measure the part clearances and timing as the action cycles, and the critical limits of those hidden details are fairly subtle.  It's easy to presume the causes incorrectly.

To narrow the subject, I am not talking about misfeeds caused by holding the rifle at an extreme angle or on its side, resulting in jams involving the bolt, breech, or ejector.  Those are a consequence of gravity and the uncontrolled feed.  I'm not talking about double-feeds caused by short-stroking or similar incomplete cycling motions. That's just a consequence of the fact that magazine interruption mechanism is reversible, while the feeding of the current round is not.  What I am talking about is the minimum reliable cartridge length for the 1894 action and jams caused by short cartridges in otherwise unworn rifles.

The Existence and Specifics of the Minimum COL Limit

But wait, the lifter automatically acts as a magazine interrupter.  Does the 1894 even have a minimum COL limitation?  Well, if you ask around, you will amusingly find people who will tell you that the design of the mechanism cleverly ensures it does not.  This is incorrect.  There most definitely is a minimum COL limitation. If you would like a quick proof that a lower bound exists, load a full magazine with empty cases (easier said than done) and see if you can get the action to lift a round without jamming on the next.

What that length is depends on the cartridge.  While the lessons of this study should also apply to other chamberings, 44 Remington magnum provides an illustrative complication as its chamber can also accept 44 S&W special. The SAAMI COL spec for 44 Magnum is 1.535-1.610 inches. On the other hand, the spec for 44 special is 1.415-1.615 inches. It's conceivable that a firearm designed around the magnum cartridge spec might not accomodate the wider spectrum of 44 special cartridge lengths.  Certainly, the first page of the manual for the Marlin 1894 states the acceptable range of cartridge OAL echoing the SAAMI spec for 44 magnum.  The case with 357 magnum and 38 special is similar.  These new boxes of ammunition are approximately 1.475" long -- well outside the limits recommended by the manual.

While the spec on paper is merely an assertion, it is the design of the mechanism and the particulars of its geometry which ultimately determine the actual practical range of cartridge lengths. When the lever is closed, the current round is captured at an angle between the loading gate, the right-hand face of the lever arm, and the next round in the magazine.  As the lever is opened, its arm clears the rim of the current round, allowing it to slip off the front edge of the loading gate.  At this point, the cartridge is captured between the forward face of the lever arm and the subsequent cartridge.  The cam surface on the lever begins to raise the lifter, shearing the stack of cartridges as they follow the motion of the lever rearward. In order to function correctly, the lifter must rise high enough to block the next cartridge before its rim clears the mouth of the magazine tube.  If the cam-lifter timing is retarded by some defect, or if the cartridges are too short, the lifter will not be in a position to fully block the next round before its rim clears the magazine mouth and a jam will occur.  It is this geometry of the lever arm and cam surfaces, as well as the lifter and magazine mouth which determine minimum cartridge length. 

Lever closed; cartridge offset and held on loading gate

Lever open; cartridges following lever arm

Magazine interrupted; cartridge uncontrolled

You should now be experiencing a creeping doubt regarding the invariability of the effective minimum COL requirement.  Not only is it an issue of timing between moving surfaces, the mechanical advantage of the cam/lifter system means that feed reliability is going to be significantly sensitive to small geometry changes caused by wear or receiver alignment.  There's enough tolerance in the screw holes that reassembling the rifle in a slightly different manner may significantly change its reliability with marginal-length ammunition.  If the receiver halves are tightly-fitted, the front screw may not pull them together completely with reasonable torque; a few taps with a hammer may be necessary to seat them completely. It's easy to think it's assembled correctly, and still have a 0.005" change in alignment make the reliability go from 95% down to 10%.  The same can happen if there are burrs or any debris between the receiver halves near the front screw.  The "Marlin jam" fixes addressing seemingly miniscule lifter peening should now start to make sense.

Very short COL resulting in jam on cartridge body

Marginal COL resulting in jam on rim

The form of the jam may vary depending on the scenario, leading people to mistakenly assume that the causes are necessarily different, perhaps misattributing them to other causes which produce identical end-conditions.  In the case of a very short cartridge, the next round is entirely trapped between the magazine mouth and the top face of the lifter.  In the case of a marginally-short cartridge, the rim of the next round may protrude only far enough to hook edge of the magazine mouth, becoming wedged against the front face of the lifter nose.  Bear in mind that the front edge of the lifter is not always close to the magazine mouth; neither is it moving in a straight line.  It moves in an arc, and presents a considerable gap when it is only beginning to interrupt the magazine. The exact contouring of the front edge of the lifter nose, caused by wear or intent otherwise, may further contribute to inconsistent behavior in marginal conditions.

Of course, this is just my insight from having spent a few hours trying to get a single rifle to run with two boxes of a specific brand and loading of cartridge.  I don't own 50 Marlin rifles.  I haven't fired 1E5 rounds of every kind of ammunition ever made without a single jam.  I haven't shot Marlins professionally for the last 90 years, uphill both ways in the snow.  I don't even hang out on the forums with all the people who assuredly did, so what do I know?

Toward a Remedy

While this particular case is an issue of short ammunition, the effect is the same as the lifter wear with which other owners contend.  As mentioned, I'm not the first person to try solving the problem; there are plenty of proposed fixes.  The bottom surface of the lifter which mates with the cam may be built up by welding or using an epoxy-bonded shim.  The lifter itself could be bent slightly. Generally, these solutions amount to a timing advance to shift the range of allowable COL down.  I don't know whether it's possible to effectively reduce the maximum COL by doing this; that limitation may be determined elsewhere in the mechanism (distance from magazine to cartridge stop surface) or loading cycle (chance of stumping on the breech face).  I'm not worried about overly-long cartridges at the moment.

The cam surface on this lifter is different than that of other (newer?) rifles I've seen.  Its curvature precludes the easy use of spring steel shims.  I simply elected to shape a brass shim from 0.015" shim stock and adhere it with JB-Weld.  This may prove to have a relatively short longevity, but it is more than adequate for this application.  With only a 0.015" shim, the lifter nose is advanced roughly 0.030", and the interruption occurs about 0.150" earlier (depending on where you assume the cartridge contacts the lever arm at that moment).  Again, the point is that small changes in the cam interface cause a large change in the effective minimum COL.

For my own purposes, I am content with the potentially temporary shim.  I don't want to weld or bend anything permanent; the shim can easily be removed without harm.  All it needs to do is allow the rifle to function with this particular lot of ammunition -- which it does excellently.  Once they are consumed, the rifle can simply be fed with a more appropriate choice of ammunition.  The long-term solution is merely an applied understanding of the mechanical nuance -- and/or perhaps some handloading.

Afterthoughts and a Helpful Gesture

It might just be me, but these 240gr SWC bullets almost look more at home in 44 magnum brass.  They would produce a good COL in that combination.  It might simply be an issue of the difference in demand between the cartridges and a desire to consolidate production components.  If everyone likes their Keith-style SWC's in the magnum cartridge, why not just stick the same bullets in the 44 special offerings too?  After all, 1.475" is well within the SAAMI spec for 44 special, and it's probably a fair assumption that most of them wind up in revolvers anyway. 

As for those who want to run lighter loads in their 44 magnum 1894's without dealing with tweaking the rifle, there are probably some factory loads available with more appropriate COL.  Of course, nobody states that metric on the box or the web.  I guess you can always bring your calipers to the store or do some approximation from image analysis.  The alternative is to handload.  Again, SWC bullets with high cannelures seem to be the most popular.  It may simply be easiest to use those with magnum brass to get a more reliable COL, loading down to 44 special ballistics.  At least at Brownell's, the price is identical for magnum or special brass -- for both Starline and Winchester.  That's probably of little comfort if you're sitting on a pile of special brass.  I suppose if casting is desired or acceptable, the Lyman Cast Bullet Handbook linked below lists the COL for cartridges assembled with various bullets from Lyman molds.  For example, a comfortable 1.571" COL can be achieved with a 245gr bullet from the #429421 mold.  There are always plenty of other recommendations around the web.

Toward that end, these pdfs may be useful.  Sometimes the older books are actually favorable if you have stock of discontinued powders, or if you want pressure data for legacy loads as a reference for comparison.  If you want newer editions, just hit up Amazon or AbeBooks, or the relevant manufacturer's site. 

Lyman Cast Bullet Handbook - 3rd Edition - 1980
Lyman Reloading Handbook - 48th Edition - 2002
Modern Reloading - Richard Lee - First Edition - 2000
Hornady Handbook of Cartridge Reloading - 3rd Edition - 1980
Hornady Handbook of Cartridge Reloading - 4th Edition - Part 1 - 1996
Hornady Handbook of Cartridge Reloading - 4th Edition - Part 2 - 1996
Cartridges of the World - Barnes - 8th Edition - 1997
ABC's of Reloading - RCBS - Dean Grennell - 1985
The Gun Digest Book of Handgun Reloading - Dean A Grennell - 1987
Gun Digest - Shooter's Guide to Reloading - Philip P. Massaro - 2014
Precision Handloading - John Withers - 1985
Introduction to BPCR Loading - Chuck Raithel 2001
A Cast Bullet Guide for Handgunners - Fryxell - Applegate
The American Rifle - Whelen - 1918
Complete Guide to Handloading - Sharpe - 1937
Handloaders Manual - Earl Naramore - 1943
Alliant Reloaders Guide - 2009
Hodgdon Basic Reloading Manual - 2015
Winchester Reloaders Manual - 15th Edition - 1997
Western Load Guide - 6.0 - 2016
Powder Bulk Density Chart - 2007