- Recently had occasion to design my setting's .50 BMG equivalent. .50 BMG is actually exactly 13 millimeters, I don't know where that "12.7×99mm NATO" business comes from. The bullets are typically a full 60 millimeters long. Now, to move a 49-gram bullet at 860 m/s requires 14.515 grams of nitrocellulose propellant, so it'd take 6.096 grams of octanitrocubane. 6.096 grams of ONC (density 2.06 g/cm3) has a volume of 2959.369 cubic millimeters.
Going with the base-area of the .50 BMG, which has a case-diameter of 20.4 millimeters, resulted in a somewhat gawky final product (the propellant "casing" went less than halfway up the bullet). So instead we're gonna go with the shoulder diameter, 18.1 millimeters. Now we just say that the cylindrical volume of a cylinder of that base-area, height undefined, minus the volume of the bullet, equals the volume of the propellant. Then we add the volume of the bullet and propellant, divide by the case area, and get a height for the "casing" (actually the propellant) of 42.45 millimeters. The "casing" sticks out 2.55 millimeters around the bullet (and out from behind it), and goes 39.9 millimeters up the side.
Hence, I guess, my Peacekeepers' sniper-round is "13×43". With an overall length of 62.55 millimeters, and a total cartridge-weight of 55.1 grams, it's a lot lighter than a round of .50 BMG, which weighs 116.8 grams. That means that your sniper can carry a lot more ammo—over twice as much. Currently, .50 BMG snipers usually carry 50-100 rounds, which weighs 5.84-11.68 kilos; 100 rounds of the caseless only weighs 5.51 kilos.
- Apparently, I was wrong, octanitrocubane would not smell like camphor. It's in the same family of explosives as RDX and HMX; the former is what's in C4. Commercial C4 has "odorizing", "taggant" additives put in to make bombs made of the stuff harder to hide (I don't think the military bothers)—the main "taggant" in the US is apparently very noticeable to dogs, I don't know how noticeable humans find it. But apparently, on its own, it smells "bituminous", i.e. tarry. Personally I like that smell, like fresh asphalt, but I'm apparently in a minority on that one.
Also, I must be more careful about specifying it's denatured octanitrocubane. ONC, see, is a high explosive; firearm propellants are low explosives—they deflagrate (burn) rather than detonating (kablooie). But, when Heckler and Koch were making the G11's caseless ammo, they (in order to solve "cooking off" issues) used a propellant consisting mostly of RDX, denatured so it would burn slowly enough to propel ammunition—and not explode in the user's hand. (The reason they went with RDX is it's harder to ignite than nitrocellulose is, hence it solved the "cooking off" problem caused by no longer having the ejection of spent casings for a heat-sink.)
- Recalculating, I find I can, in fact, have my 12-gauge round be caseless. Went with twenty pellets of #3 buckshot, the same as was used in the M576 buckshot-grenade for the old M79 grenade launcher—which isn't really a grenade, it's literally a shotgun shell you shoot from a grenade launcher. But I decided to make the pellets out of the same tungsten alloy as those found in the QBS-09; taking their diameter of 5.3 millimeters and their mass of 1.4 grams, we get a density of 17.96 g/cm3, which makes #3 buckshot (diameter 6.4 millimeters) weigh 2.465 grams—twenty of them mass 49.3038 grams. I'll come back to that.
Decided to make the propellant straight nitrocellulose, not denatured ONC, for this one: we want the propellant to take up room. The loading tables for a 49-gram load say it takes 2.0088 grams of powder; that, in nitrocellulose (density 1.40 g/cm3) has a volume of 1434.857 cubic millimeters. Now, the closest you can pack spheres still results in wasted space—the maximum efficiency of packing equal spheres is 74.048%. Twenty pellets of #3 buck has a volume of 2745.166 cubic millimeters; in effect, though, it has a volume of 3707.279 cubic millimeters, wasting 962.113 cubic millimeters. We just have to fill that in, though, with our propellant. The remaining 472.744 cubic millimeters? We divide that up among the twenty pellets, solve the resulting volume for its radius, and come up with a coating .17 millimeters thick. Interestingly, #3 buckshot with that coating exactly comes to the edge of a 20-millimeter diameter circle when you pack it hexagonally, as if this arrangement were fore-ordained.
You stack your twenty pellets in one layer of seven, one layer of three, one layer of seven, one layer of three. I checked, it fits. This arrangement results in a round 24.29 millimeters long, and, again, 20 millimeters in diameter; the remaining volume around the pellets and propellant is presumably filled up by some kind of flammable filler—"liquid wadding"?—to make up the rest of the cylinder. (I envision it being a clear, resinous-looking substance, I'm not sure why except that it'd look awesome.)
- Now, it turns out I miscalculated before, on the number of rounds a shotgun can hold; shotgun-shell lengths are apparently the length of the casing before being crimped down (or after firing), and are about half an inch longer than the actual length of the round. So 2.75" shells are really 2.25 inches long; a QBS-09's magazine is 28.575 centimeters long, the Benelli M4 and Remington 870's magazines are 40.005 centimeters, and the Mossberg 590's magazine is 45.72 centimeters. That means the QBS-09 can hold eleven rounds (nearly twelve—its 24th-century equivalent probably the full dozen) of our caseless 12-gauge round, the Benelli and Remington can hold sixteen, and the Mossberg can hold eighteen (nearly nineteen—the full nineteen, again assuming a generous future).
- A shotgun slug with a weight of 49 grams isn't all that unusual, though that's kinda a big one (1.75 ounce). But it requires the same amount of nitrocellulose the buckshot does; with this one, we can go with ONC. I find a typical 12-gauge slug is about 17.65 millimeters in diameter and 21.59 millimeters long. That requires (2.0088/2.38=)843.696 milligrams of ONC, which has a volume of 409.561 cubic millimeters. Treating the slug as a cylinder, we find that the denatured-ONC propellant "casing" sticks out from it 1.175 millimeters, and goes 16.94 millimeters up the side of the slug. That brings its total length to 22.765 millimeters, meaning it pretty much fills out shotgun magazines the same way its buckshot counterpart does (you could carry one extra in a Benelli, Remington, or Mossberg).
Presumably the shotgun grenades would have the same mass as the slug, and the same amount of propellant. There's basically no easily available information on the 20 mm kind, but I find that an "air burst" 40 mm grenade contains 32 grams of OCTOL (air-burst seems to be the main thing the OICW would've been used for—same seems to go for the less-experimental, or at least less canceled, Daewoo K11 and Chinese ZH-05). Now, ONC (not denatured this time) is six-elevenths more effective than OCTOL, so to get the same explosive yield requires only 20.71 grams; the rest is the casing and fuze. The armor-piercing type of 40 millimeter grenade requires 45 grams of Composition A; ONC is 48.75% more efficient than Composition A, meaning it'd take a full 30 grams to make an AP grenade (presumably they can miniaturize the fuze and make an ultra-light casing, it's the 24th freaking century).
- Do you remember my coil vulcans? They use the "gatling" mechanism to reduce wear on the coils. Well, it occurred to me, the likely ammunition for that gun, whether vehicle-mounted or carried by powered-armor troopers, is something like the Raufoss Mk. 211 High Explosive Incendiary Armor Piercing round, which uses a steel core and a tiny amount of RDX to accomplish, in a .50 BMG round, effects you usually need 20 mm to achieve. Of course, being a coil-gun round, some of it's going to be different—it'd probably have at least a small amount of exposed iron, for instance (maybe a "driving band" like on artillery shells?).
Average troops equipped with lifting exosuits could use it with a tripod, maybe, though I don't know of anyone who's used the GAU-19 (chambered in .50 BMG) that way—and the Soviet Yak-B machinegun is only used on the Hind. You would need powered-armor troopers to use the thing without a tripod; the recoil of the GAU-19 is 2.2 kilonewtons at 1,300 rounds per minute, but goes down to 1.7 at 1,000 rds/min and up to 2.8 at 2,000. It takes about 2.7 kilonewtons to knock over a 113-kilo man (a basketball player, specifically, in what's basically a horse-stance); therefore, it takes only 1.7 kilonewtons to knock over a 70-kilo man in the same posture (not that you stand like that while shooting a machinegun). Presumably for handheld, or even mere tripod applications, you'd keep the rate of fire at 1,000 rds/min.
A thousand Raufoss-equivalent rounds would weigh 43 kilos; each link of a disintegrating belt weighs about 2 grams, a thousand of which is another 2 kilos, bringing the total to an even 45. The GAU-19/B weighs 48 kilos, so you really want that power-lifting exosuit—which may also be necessary to using the thing by hand, I wouldn't be surprised if the reason nobody uses .50 BMG tripod guns now is the sheer difficulty of pointing the thing. (By going with titanium instead of steel, the links could weigh as little as 1.1 kilos; and if we use polymer like the LSAT—which you may have to, with caseless—1,000 links weighs only 500 grams.)
- One thing I realized, since my setting's "assault rifle" is chambered in something that approximates .30-06: they don't need much special equipment to be a "designated marksman". So, in my setting, there is no DMR; they just attach a bipod and scope to the standard assault rifle via its accessory-rail.
Since it's a bullpup AK-type assault rifle, I guess it looks like the Polish Jantar carbine (prototype/demonstrator). Or, even more, like the Kalashnikov Concern AS-1 and AS-2, of which there seems to be, more or less, one image on the whole internet. (Your guess is as good as mine why they're green.)
While I'm at it, the bullpup ARs my USMC uses, are basically like this, the K&M Arms modernized Bushmaster.
- The main underbarrel grenade-launcher used in my setting is also a shotgun, like a cross between the M26 Modular Accessory Shotgun System and the M576 buckshot grenade in reverse.
It occurred to me that my Peacekeepers would classify all their shotguns as grenade-launchers—because it's no longer the least bit remarkable that they can be used for 20 millimeter grenades. It's not that uncommon to classify things oddly, in militaries; several Russian shotguns are designated as carbines, and not all of them have rifled barrels like the KS-23. Or how about the US's pathological aversion to the words "light tank"?
I'm not sure how prevalent 40 millimeter grenades are, for my Peacekeepers, though zledo use their equivalent here and there. I suppose the humans probably prefer the 20 millimeter ones for the high end of "small arms", and use the 40 millimeter grenades for their equivalent of the Mk. 19 grenade machinegun (and its successors).
Les armes à feu spéculatives 3
Thoughts on SFional guns.