A brief digression into advanced conventional gun technology in GURPS Ultra-Tech.

There are two advanced technologies for firearms in GURPS Ultra-Tech. Liquid Propellant slugthrowers improves piercing damage by +1 per die (about 28%) with the boosted velocity version, and there’s a low velocity option that cuts damage in half, but we don’t care that much about that. Electrothermal-Chemical weapons increase damage by 1.5x. There are also Electothermal-Kinetic weapons, discussed in a Designer’s Notes for Ultra-Tech, Fourth Edition article, which are even more badass, multiplying damage by 2x.

What does that mean in terms of weapon design?

The Ballistics Model

My model – and remember, all models are wrong, some are useful – takes the peak pressure and maintains it for a certain distance down the barrel. This adds Pressure x Barrel Bore Diameter x Burn Distance in energy to the bullet: a straight-up application of energy equals force times distance. After that, it assumes that the propellant is all burned, and that the chamber and barrel and bullet form a pressure vessel, with the instantaneous pressure calculated using the ideal gas law. One can then calculate the instantaneous force at a given length down the barrel, and sum the product of that force over the distance – you’re performing an integration: E = Integral of F dx.

That’s wrong.

It just is. It’s not unusable, but it’s a simplification. It neglects the non-ideal component of the pressure equation, and it really ignores the fact that as the bullet is driven down the barrel, there is an opposing force of friction acting to retard the projectile. So while in real life, there’s an optimal barrel length after which more barrel is a hindrance to velocity, my model assumes that the bullet will continue to be accelerated pretty much forever.

That it falls off over distance, though, means that you need more and more barrel to actually get a GURPS-useful boost in velocity. So in practice, while it’s “wrong,” it’s not so wrong by so much that it doesn’t provide a useful and self-consistent model. Perfect? No. Usable? Yah.

But given the way that the program works, there are really only a few ways to add energy to the projectile.

Increase the pressure

This is the obvious one. A higher starting pressure will result in higher force the entire way. Using the M16 firing an M855 bullet as an example, if I increase the pressure from 55,000psi to 66,000psi, a 20% boost, the energy goes from 1776J to 2130J, unsurprisingly a 20% boost in energy.

The hard part about increasing pressure is that it’s hard on barrels and breech-locks and cartridge cases. It’s an engineering challenge, though not an insurmountable one. The new M855A1 “green” cartridge that replaces the M855 pushes pressure to 63,000 psi, which is known to cause barrel wear issues in M4 carbines. The Mk318 Mod 0 that the USMC adopted pushes to just shy of 60,000psi, but that’s not apparently enough to produce noticeably accelerated wear, so I am given to understand.

Increase the burn distance

The pressure is held constant for a bit, representing the powder combusting and staying at max pressure. One can assume more powder that burns evenly and slowly for a longer distance. The burn distance is one of the great fitting parameters in the model. Knowing the pressure (which one can usually find out) and the barrel length and other things about the bullet, such as the muzzle velocity, you just use the burn distance as a fitting parameter to make things work out to a known reference.

The M16 above has a burn distance of 11.1mm, which gives the required 940m/s out of a 508mm barrel. To increase the energy by 20%, that distance has to increase from 11.1mm to 33.9mm. That’s quite a bit, but it’s still less than the length of the case, and most bullets’ burn distance is roughly half of the length of their case (pistols tend a bit less, rifles a bit more).

Increase the chamber size

This one’s a bit odd, but if the starting volume of the expanding gas is large, then the relatively small addition of volume “created” as the bullet goes down the barrel is a relatively small adjustment in pressure, so the force will stay close to peak because the change in volume is low, so the change in pressure is low, and force is pressure times barrel area. That initial volume is called the “fall-off” volume in my model. Doesn’t really matter why. That volume is the volume of the chamber, plus a bit of barrel, though thinking about it perhaps I should have subtracted that instead of adding. No matter.

For our M16 example, the starting volume is 2.566 cubic centimeters. How much larger would it need to get in order to increase the energy by 20%? Turns out it’s 3.794 cubic centimeters, which is a chamber diameter increase from 9.3mm to 11.5mm – that’s not insane either.

Increase the bore diameter

So if energy is force times distance, and force is pressure times area, then you can always just increase the area that the force is pushing on. Say, increase the barrel diameter from 5.7mm to 6.8mm, which, no magic to it, is also 20%.

Now, you don’t increase the bullet diameter, just the bore. That means a sabot, and in practice, you’ll be increasing it a bit more than 20% because you’ll lose some energy in the sabot, which falls away.

ALL THE BOOSTS

You can, of course, mix and match these. If you want to go nuts, drive a .223 bullet from a chamber that’s 11.5mm in diameter, at 66,000psi, with a 33mm burn distance, using a 6.8mm barrel with a sabot massing 7 grains (a bit more than 10% of the projectile weight).

What happens then? You fire a 55gr 5.56mm projectile (like the M193 bullet) at 1,350m/s (4,430fps) instead of the bog-standard 940m/s. If you’re thinking to yourself – “hey, you just re-invented a slightly up-gunned version of the .22 Swift,” well . . . you’re not wrong.

Liquid Propellant

The short version is that the +1 per die boost is a 64% increase in the kinetic energy of the projectile, and a 28% increase in the velocity of the projectile. That’s quite an increase: If (for example) you were taking a 20″ barrel M855 bullet (the old standard semi-AP M16 projectile), you’d expect the energy of this to increase from 1,776J to 2,935J. That’s non-trivial, but can probably be done.

Just jacking up the pressure will be a whole lot, and the usual 55,000psi would have to go up to 90,000 in order to make that happen. That’s asking for trouble.

Just cutting to the chase here, though, think about it. We need a gas bottle (technically, we need two – propellant and oxidizer) and we’ll inject that with a system not unlike the fuel injector of a car into a fixed combustion chamber. We don’t need a cartridge to carry the propellant – that’s integral to the weapon. We can use a spark plug instead of a mechanical hammer (though a mechanically actuated spark is probably worth considering for reliability reasons). In short, the fairly vast area inside the lower receive of an M16 can become the chamber. The magazine for the rifle now more or less just needs to hold the bullet itself, which means that they get shorter and could easily hold 250 shots in a small space: four wide is less than an inch thick, and 60 tall is about 300mm, or about a foot. The length of a bullet is about 1.5″ long, which means that it could easily be mounted on top of the weapon, G11-style, looking a bit like the stick magazines of the M-21 Thompson of WW2 vintage.

A compressed gas bottle might need to be replaced every few magazines, or might even be the rate-limiting step. But basically you can probably get away, in an LP rifle, with simply getting your energy out of really big combustion chamber, and also taking advantage of the sabot concept, while lowering pressure and having a modest to zero burn distance.

An example? Let’s say the combustion chamber is 30mm in diameter, and 60mm long. That’s a bit skinnier and a bit longer than a D-cell battery (a real one, not an Ultra-Tech one). Burn length is set to zero – the chamber detonates and that’s it. Barrel diameter is 6.8mm as calculated above. We’ll increase the total accelerated mass from 62gr to 70gr and assume an 8gr sabot, to keep the projectile the same as the bog-standard M855.

But . . . we’ll lower the pressure to give that M855 the same energy as today: 1776J.

The required pressure drops from 55,000 psi in an M16 to 16,500psi in our notional liquid propellant rifle.

Well, well, well. That’s shotgun level pressures (well, no surprise, since the chamber is shotgun sized). Hmm – let’s try that. We’ll go with an 18.5mm chamber that’s 2.75″ (70mm) long – a 12G shotgun shell. The pressure creeps up to 23,400psi to get the same velocity out of the 62-grain projectile. Still very low by assault rifle standards.

But freeing yourself from the confines of a cartridge that is the pressure vessel, and using a compressed binary liquid propellant in a fixed chamber (actually, some parts of the chamber, be it the front or back wall, or just enough of it to allow the next bullet to be loaded, will have to move) will result in a weapon that is more powerful if you want it to be while at the same time holding quite a few more shots per reload.

Of course, the calculations above were for equal performance to the M855. To get +1 per die, you need to go from 1776J to 2,935J. The pressure required to do that in out shotgun-sized chamber is . . . about the same found in a 10mm Automatic pistol: 38,700psi. Of course, that’s quite a bit of force, but you know it, you have a lot of potential mass to work with in the weapon itself. Should be doable. The real question is how compact and how light you can get (say) 500-750 shots worth of binary propellant, so you can fire through 2-3 magazines worth of bullets (at 200-250 shots per magazine!) before you have to change both the projectile and the bottle.

Note that selectable fuel injection means that you can certainly get thousands of rounds of “regular” grade velocity, and even more so subsonic velocity, by dialing down the gas injection. Having a dual-feed, top-mounted pair of magazines so that you can (say) choose between a heavy, possibly AP bullet for subsonic or long-range shots, and a standard bullet for suppression fire? Again . . . should be doable.

Electrothermal-Chemical

The ETC weapons are a bit different than I had written before this was updated; I had conflated the ETK and ETC systems in my mind.

The Electrothermal-Chemical system uses an electrical charge and a plasma-forming medium to enhance and prolong the combustion of a propellant. This means a few things: that it will usually be a cased system, and so the usual restrictions on magazine size and chamber volume still hold. It will also require a battery or some other source of power. Much like the Liquid Propellant option, this means that there are two things to deplete, rather than one, making for potentially longer magazine change-outs unless the battery is integrated with the magazine; engineering analysis would need to prove out whether “bigger cartridge” outweighs “regular cartridge plus battery.”

Finally, a look at the pressure profile of ETC weapons shows that within the context of my model, the pressure is not much lower, so what ETC does is increase the burn distance. For tank guns, clearly the sabot principle applies, but it would also appear that the best way to do this is to simply increase the “burn length” parameter until the kinetic energy of the final projectile increases by 100%, to 3,550J. In my model, this requires a burn length increase from 11.1mm to 167.5mm, a 15x increase. This is still less than the 508mm barrel, so as long as some effort is taken to control muzzle flash, a round “increase burn by 15x” is fine. For example, an ETC version of a .50 BMG (whuff) would need to hit about 34kJ, and a 19x increase in that parameter gets us there. Being conservative and saying apply a 20x increase might cover the bases. Still, there’s plenty of barrel left. Maintaining maximum pressure the entire length of the .50BMG will give 47kJ from a start of 17kJ, so about 2.5x; the M16 goes from 1.8kJ to 4.5kJ as an upper limit without changing the chamber pressure, chamber volume, or messing with sabots.

For battery size, I have no idea how much of the input energy comes from a battery, and what comes from more complete and prolonged powder burn; reports say that most of the energy comes from the propellant. Let’s say a third to half of the extra energy comes from the battery, which for our M16 example is on the order of 600-900J per shot from the battery.

Since this is basically expensive modified ammo and an extra battery, we look at what 30 rounds x 750J looks like: 22.5kJ for each magazine. A lithium battery is listed in Wiki with an energy density of 500 W-h per kg, or 1800 Joules per gram. Even if I look at more conservative numbers (I’ve seen 150 W-h per kg floating around), you’re still looking at 500J per gram, which means our requirement, with TL8 battery technology, masses about 45grams, which is little enough (about an ounce and a half) that you can put the energy required into each magazine and make it disposable. You can also put a larger battery (say, 1 lb) into the rifle itself and fire off 7 magazines (300-500 rounds) before you have to load a new 1-lb battery. If you make the battery rechargeable, you might even be able to plug it into a portable generator (microturbine at the squad or fireteam level), and liquid and gaseous fuels are very efficient for their weight in this way.

Net of all this, though, is that while you will need to worry about a battery, more expensive ammunition (the plasma generator part of the ammo), and likely deal with flash and noise (integral flash/sound suppressor will do this handily, adding maybe a few inches and a pound to a rifle-class weapon), the assumed +50% damage (+100% energy) is achievable.

Electrothermal-Kinetic

The technobabble (I say that in the nicest possible way) here takes plasma-driven combustion in a firing chamber that is driven by electrical energy input, and then supplements it with still more added electrical energy as the bullet goes down the barrel. And you’ll need it: to get four times the energy out of the projectile, which doubles damage, you will need quite a bit of oomph.

Let’s take the prior design and run with it – what happens to our energy (goal is 1,775J increases to 7,100J delivered) if we keep about 55,000, and use the same chamber, but increase the burn length to the entire length of the barrel? Nope. Can’t get there – the max is 4,500J and we still have 3,500J to go.

Note that in this case, if we’re going to pump energy in and keep the pressure at maximum no matter what, the chamber volume is not relevant (there’s no point in a pressure reservoir here). In this case, a straight-walled or otherwise narrow, short cartridge that basically packs inert working material around the bullet is all you need . . . but you will need to amp up the pressure to get what you want out of it based on my model assumptions (and at this point, I’m questioning them).

To the meat of it: you need to boost the pressure to 82,625psi and hold it there for the full length of the barrel to make this work.

Or you boost the pressure to a not-insane 63,000psi (that’s the current pressure of the M855A1 cartridge) but give an extra 3-6″ of barrel, 23-26″ instead of 20″. Call it a 15-30% increase in barrel length. With a bullpup configured weapon that’s still reasonable, giving 8-12″ behind the barrel provides for a stock and mechanism would be a 31-38″ weapon, not a carbine by any means, but still 2-9″ shorter than an M16A4 at this point in the design process.

You will, however, have some very excited gas that stops having energy dumped into it as the projectile leaves the barrel. So we’ll tack on a sound and flash suppressor that’s integrated with the barrel. This will add some weight (call it a pound) and length (4-5″ of barrel length that’s not used for accelerating the bullet), which will bring the overall weapon solidly into the rifle class, anywhere from 2″ longer to 7″ shorter than the M16 (call it 3″ shorter as an average), but certainly you won’t be dealing with “short barrel” issues.

Finally, you’ll have the fact that a relative long weapon will have a relatively long sight radius and good accuracy potential. Acc 5 or DMR versions that are Acc 6 are quite feasible here.

By scaling up the M16 to double damage, you have a 10d rifle based on a rather old bullet concept. Improve it a bit with (say) a 6.5mm projectile with a 5:1 aspect ratio and a bit of optimization, and you’re throwing a 12d standard projectile with a 1/2D range of 650yds with standard construction, and over 1,000 yds with a high-density construction. Marksmanship fans rejoice.

On the flip side, you’ll need a lot of battery with this, since all the power (and some waste) will come from the battery itself, with little provided by the weapon. If we assume 1000J per gram of battery, each shot will need about 8-10g of battery mass. So a single magazine is pushing 1 lb of battery alone for 50 shots. On the other hand, just looking at the projectile and some sacrificial mass, maybe 10gram per bullet, that’s another 500g (1 lb). So projectiles plus battery is a 2-lb magazine. The typical magazine weights already in UT are 1.5-2lbs for this class of weapon, so it’s not that out of whack.

Parting Shot

I’ve long been skeptical of the ETC weapons in particular as being too much of a good thing. That may still be true. But with careful weapon design, and looking closely at the benefits of each potential source of extra force on the projectile, I’m fairly sure that such a weapon could be built.

The even-more potent ETK rounds are a bit more of a stretch, but with proper design and a big battery

I don’t know if the promised recoil reduction would manifest, though. Force is force, and given a constant barrel length, to get more velocity and GURPS damage (but I repeat myself) out of a bullet, you have to push harder, and that means the weapon pushes harder on you. 

Physics is a bitch that way.