The M41 Pulse Rifle – stats and commentary

Hicks: I wanna introduce you to a personal friend of mine. This is an M41A pulse rifle. Ten millimeter with over-and-under thirty millimeter pump action grenade launcher. 
Hicks: [Hicks hands the rifle to Ripley] Feel the weight. 
Ripley: Okay. What do I do? 

The M41 is iconic. It defines the look and feel of one of the best Aliens movies out there (in my opinion), and was, at least in this scene, treated more like a character than a piece of equipment. 

I recently wrote up the ammunition it supposedly fires as my April Fool’s entry for The Reloading Press. It’s described in the movie as follows:

Ripley: Lieutenant, what do those pulse rifles fire?Gorman: 10 millimeter explosive tip caseless. Standard light-armor-piercing round, why?

Other than the weapon itself, which was made in several non-firing mock-ups and one “hero” weapon that could actually fire blanks, that’s mostly the only information you have on the weapon itself. You do see that loading a fresh magazine gave something like 95 or 99 shots.

This writeup is dedicated to +Kyrinn S. Eis who asked me very nicely to do it.


Screen to Reality Problems


The weapon itself was converted from a .45 Thompson SMG, because the director liked the “angry” flash of the .45ACP cartridge. He wanted the weapon to be impressive when fired.

It was highly modified, obviously, and featured a slung-under grenade launcher that was obviously a converted pump-action shotgun.

It’s got a short barrel for both weapons, making it definitely in the SMG category.

Some of the issues that come up right away have to do with the quoted dimensions of the projectiles. A 12G shotgun is 18.5mm in diameter. The quoted dimension of the grenade launcher in the movie is 30mm, which is nearly twice the bore of the mock-up weapon. As you can see from the UMP in the picture, a 30mm grenade launcher would have been unmistakable. The grenades just aren’t that big.

The second problem – and that’s even before we get to the Colonial Marines’ Technical Manual information – is that the stated 10mm bore size of the rifle runs into issue. Not with the observed barrel opening – that’s probably fine, as the Thompson is, in fact, .45ACP (11.43mm) and larger even than the 10mm caseless claimed.

No, the problem is magazine capacity. A 10mm bullet is . . . wait for it . . . 10mm in width. So if you stack them like they’re in an M1911 pistol (which you’d never do), then 95 of those would be almost a meter long. Basically a three-foot-long magazine. 

Even the 25-round magazine of the UMP above, which protrudes so conspicuously beneath the weapon, is probably about 25cm long, and going from .45 to 10mm might drop that by 10%. Now, it is deceptive – the distance from the barrel to the bottom of the magazine well in the picture is longer than it would appear. And a better graphic shows it as well.

But ultimately, if the magazine is as shown, it will maybe be 7-9″ long – or about 200mm, maybe 225mm. That allows for a maximum stack of 20-22 bullets per column. Let’s be generous and say 25.

Now, you can offset them in width, of course. But ultimately, to get 90-100 bullets out of that magazine for that size, you need four to five columns. Even staggered, four columns will be 36mm wide. Or about 1.4″. Five wide would be 45mm, or 1.75″.

Mostly, though, the magazines are presented as a double-stack configuration, as shown in the replica to the right.

But . . . there’s at least enough room for one more column to the left and right of the double-stack. The movie didn’t show that, but then, there aren’t many quad-stack magazines kicking around for a .45 Thompson. There are plenty of quad-stack mags for assault rifles, though. Well, if not plenty, than some. And since the typical base dimension of an assault rifle round is 9-10mm, having a quad-stack in 10mm . . . even 10mm caseless, isn’t crazed.

OK, so quad-stack. Go us. We’ll still need 24 or 25 bullets tall, though, to get as much as we’d like (95-99 rounds) out of the weapon. But if we allow 200mm tall, perhaps we can get 20, and assume that the spring (if they still use springs in the Colonial Marines) is in the large baseplate of the magazine. In fact, there’s little other reason, except maybe a battery for electric ignition, to have such a large baseplate. That gets us to roughly 80 rounds, with some degree of loss due to poor packing density as four columns go down to two goes down to one.

So maybe 60-70 rounds. The Soviet quad-stack shown was 60 rounds of 5.45x39mm, and that’s fairly respectable, if complex.

The final bit of question comes from a translation from the original source material. The Colonial Marines’ Technical Manual describes the M309 projectile fired by the M41 rifle as a “10x24mm caseless” round. 

Now, that notation is usually something like 9x19mm, where the 19mm is the case length, and the 9mm is the bore of the weapon. So a 10x24mm caseless bullet should have a 24mm area of propellant, and then a protruding bullet. The wonderful (but fake) replicas created by a cosplayer (presumably) with mad skills and plenty of time on their hands seems to do pretty well. The bullet itself, if it’s of similar proportion to a 9mm bullet, will be about 20mm long; a filled explosive bullet may well be longer and lighter density. 

So can the bullets above – 10mm in diameter, perhaps 25-30mm long, explosive filled – be dimensionally correct? The CMTM gives a 210grain bullet, 10mm in diameter. A jacketed lead bullet of 4:1 aspect ratio (like the .308) will have a density (with my program) of 9.3 to 9.9 g/cc. This 210gr, 10mm projectile with a 3:1 ratio – long for a pistol bullet, short for a rifle – will have a density of about 8. It could maybe be stretched to 7.5 (a bit lower than steel) by stretching the aspect ratio to 3.2:1, but any longer than about 3.3 or 3.5 and your bullet is longer than the overall cartridge will be.

Still, the dimensions of the projectile are surprisingly reasonable.

As for the rest, the rifle is supposedly designed around a 247mm barrel, and the projectile develops 840m/s from this length. At high pressures of around 60,000 psi, this powder will remain burning for quite a while down the barrel relative to other designs in my model. So the flash would be impressive. As seen in the movie.

For GURPS, a short barrel like that would likely be Acc 4. 1/2D for a honkin’ projectile like that would be about 425yds, and about a 3,600yd maximum range. Damage at the muzzle would be an impressive 8d-1 pi+.

Oh, and of course, it explodes. I suspect that what this means is that torso hits are treated as vitals hits if it does explode (there’s a note that it doesn’t do well when not fired at body armor in the CMTM), and we pick up a (2) armor divisor due to a penetrating tip or something. RoF is single shot, 4-round burst, or fully automatic.

Recoil should be a beast. The bullet is akin to a .338 Winchester Magnum or .300 Win Mag, which should carry a Rcl of 4 or even higher. If the weapon uses advanced recoil compensation or stabilization, perhaps one could get it down to 2 or 3 (2 is what you’d see from the movie, with the weapon fired from fairly loose grips, which you’d never do with a .308 under full-automatic fire or even burst fire – the M14 was a large weapon and not exactly known for stable full-auto fire.

Rcl that high makes full-auto fire fairly pointless in GURPS; even Rcl 1 or 2 it’s hard to hit with more than one shot. I bet this thing is pretty nasty for suppression fire, though.

Parting Shot

Ultimately, the M41 is a movie weapon that someone took some pains to try and wrap some statistics around, and produced an up-gunned weapon that kicks as hard as a sniper rifle but fires out of a SMG-size weapon. From the hip.

We haven’t talked at all about the grenade launcher, of course. Alliant Tech Systems (now Orbital ATK) tried to get good performance out of a 20mm grenade to no avail – not enough fragmentation, but then they weren’t working with TL9 or TL10 explosives, either. With the right ammuntion, direct fire HEAT from a 20mm weapon isn’t entirely stupid, and with enhanced explosives from the future, is likely the best thought-out part of the entire system.

GURPS does have a pulse-rifle equivalent, by the way, already. Check out the Storm Carbine from GURPS Ultra-Tech.

And of course, while I might cast doubt on the particular combination of velocity, projectile dimensions, and technology, the weapon itself is completely viable. After all, someone went and built one!




6 thoughts on “The M41 Pulse Rifle – stats and commentary

    1. 225mm would be about 4,500J, while the 247mm barrel given in the Tech Manual is 4,800J. That puts the small one at .300 Win Mag level, and the larger barrel a bit shy of .338 Win Mag.

      I really should add the commentary as to why a 10mm explosive is a poor choice, but I had to do work.

  1. "Ultimately, the M41 is a movie weapon that someone took some pains to try and wrap some statistics around…"

    I can relate to that!

    Fantastic article; inspires me to get back on point with my own similar stuff.

  2. Sweet Jesus, I've fired a Thompson before, and I had a hard time keeping a second or third round on target. This guy makes it look like it's got the kick of a .22.

    I assume he built his out of a Tommygun as well. That's what the comments all hint at.

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