Sunday is GunDay: GURPS Ultra-Tech Rifles

The rifle is the mainstay combat smallarm, given to every trooper. The current situation in the USA and the rest of the NATO countries have the primary projectile being the 5.56x45mm cartridge in various forms, and the “rest of the world” usually either uses the heavier and slower 7.62x39mm cartridge from the AK-47, or the small-caliber upgrade that is the 5.45x39mm from the AK-74. All of these more or less eke about about 5d: The 5.56x45mm M193 cartridge (the 55-grain/3.5 gram version) basically defines what 5d is in GURPS (and also happens to be about right – that bullet/rifle combo – the 55gr projectile at about 1000m/s from a 20″ barrel – punches through about 1/4″ of mild steel). The AK-47 is given 5d+1 in High-Tech, and the AK-74 a “downgrade” to 4d+2.

My own calculations are slightly different here, but only slightly. The 7N6M 5.45x39mm is a 53gr projectile at 880m/s, for 16.8 points of damage: 5d-1 is probably the best fit here; the penetration of 6mm of armor plate at 300m that is claimed (and the Rockwell C core hardended to Rc 60) might speak to a 1.5 armor divisor if such were still used. The AK47 bullet comes in at 19 points of damage, or 5d+1, as listed, at 122gr and 722m/s. My M193 comes in hot based on energy and diameter – it should penetrate for 19.3 points of damage, but empirically does not do so; some quick calculations show that the bullet is probably operating at close to it’s maximum coherence energy, so much kinetic energy in the projectile that the bullet itself can’t take the impact. Either that or (which is more likely) my formula is off by a bit. Either way, all of these come in at roughly 5d.

All that being said, what is enough injury? 3d injury is enough to take a regular Joe with 10HP and have them risking a KO; 6d is a death check. If your foes are human – and that’s the default assumption here – anything more than that is for two things: hitting at range, because you need to reach out and touch someone there, or punching through armor.

For the second, in GURPS, the best way to get armor defeat into the design is by bullet construction, that is to say, armor divisors to knock the protection down, followed by enough kinetic energy to defeat the remaining DR and the threat. For range, again, bullet construction is key, as high aspect ratio projectiles will hold their velocity (and thus damage) better. Retaining full ballistic performance (in terms of the GURPS 1/2D metric) to somewhere between 300 to 500m seems to fit the bill for the man-portable battlefield weapon category, and that’s basically the sweet spot for the 5.5mm type weapons, and the 7.62x39mm also fall in that range – my calculations put all three in the 400yds 1/2D range. That’s because of the relatively low sectional density of the 5.45mm round, and the high cross section of the 7.62mm one.

Battlefield Personal Weapon: Goals

So, the goals here for evaluating the old designs, and developing new ones, will be to improve on the 1/2D range (which also means that velocity loss by using shorter barrels will be somewhat mitigated) into the realm of the older battle rifle range (500-700yds), provide for a (2) armor divisor through bullet design (which will almost certainly mandate some sort of high-density core), and deliver 3d-6d injury after penetrating body armor.

The question of how much armor seems a legit one: modern trauma plates with a woven fiber armor backing will frequently provide about 10d worth of protection – DR 35 – from the plates alone, backed by the woven armor. The typical threat that this is gauged against seems to be the .30-06 M2 AP bullet, which hits for about that penetration.

So any design that’s notionally TL9 should probably stand up well against that level of ballistic threat.

Special Applications Rifles: Goals

The other set of weapons that will exist are what one might call rifles for special applications. Sniping, anti-materiel usage, and payload weapons all fit the bill here. These will put a premium on accuracy, as their usually hefty ammo does not lend well to carrying a lot of it. It will also be a place where versatility will come into play, as the projectiles start to get large enough that explosive, homing, and other special devices become practical and perform well.

Let’s Get To It

That’s a lot of pre-amble: let’s see how the rifles in the book do.

Ultra-Tech Standard Rifles

The book as written provides seven rifle-class weapons. All are assumed to be caseless, and they come in a few flavors. There’s a 5.7mm weapon that fires the same projectile as the PDW discussed in the prior write-up. There are two 7mm designs, two 10mm, a 15mm anti-materiel rifle, and a 25mm payload rifle. Let’s break ’em down.

Anti-Materiel Rifle, 15x103mm

This is the slightly-bigger brother of the .50BMG, which is a 12.7mm variant; one could find a better match with the 14.5mm Soviet-era KPV projectile, but converted to a caseless platform. This is a very large projectile, and a very large cartridge containing it. the book lists the 15mmCL as 0.2 lbs (1,400 grains, or 0.09kg); the projectile for the 14.5x114mm KPV tends to be on the order of 920-1,025 grains for the bullet alone; the overall round is in the .2 kg class (.44 lbs, or 3,100 grains).

So this is likely one of the places where we just have to abandon the notional cartridge specs if we want sense. The usual load seems to be about 29g of propellant in a cartridge that’ll hold 42.5g of water. Note that the REF of guncotton is 1.1, and PLASTEX-B is a whopping 4.0; octanitrocubane as an explosive is only just shy of 2.4. So there’s a LOT of potential for inert elements in a caseless round, or even a combustible inert “case” that transitions to a propellant/stabilizer mix.

Why do we care? Just trying to figure out how much case and block we need. Polystyrene and other plastics can be approximated as the density of water – a gram (15.43 grains) per cubic centimeter or milliliter. Most explosives seem to be in the 0.9 to 1.5 range as well. So if we actually say we have 1400grains of total weight and a 15mm projectile with about a 4:1 aspect ratio, well, that’s only a density just lower than steel overall. Perhaps that’s not bad – just enough jacket to take the rifling, and a basic steel projectile. It’s not a great projectile design, but it’s not insipid.

If we do that, then the overall bullet is 60mm long. If we make the ammo a 5mm plastic-equivalent, propellant laden case around the bullet (telescoped ammunition), that provides about 19 cubic centimeters. We’d like to shoot for roughly the same volume as the powder – about 40cc, so that means an extra 43mm of propellant below the projectile. Effectively we have a 103mm total cartridge length, which is 1cm shorter than the KPV case, and 53mm shorter than the overall cartridge.

That means we define our notional round as about a 15x103mmCL, and we have plenty of boom in there, potentially achieving insane pressures.

The rifle itself is Bulk -6, which is the same as the AI Artic Warfare rifle, which is about 46.5″ long. The Barrett M95 is a bullpup chambered .50-cal that is 46″ long . . . so about right. That one has a 730mm barrel, so let’s say our notional Anti-Material Rifle is about he same size as the AI AW, but configured like a Barrett M95 with that same barrel length? Well, it is twice as heavy as the AW, but the conventional layout M82A1 is -7, so I think we’re OK with that length despite the weight.

That means that with a very modest pressure (50,000psi), a decent burn distance, and a 730mm barrel, we can launch that projectile with enough velocity (2850fps, or 868m/s) to hit the listed 15d damage point. That doesn’t really require any Ultra-Tech assumptions other than the combustible-case telescoped cartridge itself.

The stats for this one should be closer to 925yd 1/2D range, and 6,400 yd max rather than the 2,000/9,000 listed. Acc 6+3 is exactly that of the Barrett, so that’s not even a stretch. The ammo is about 5mm larger diameter than the .50BMG, and the M95 only weighs 23.5 lbs empty. a 10-round magazine with ammo only is about 2 lbs, and so a 2.5-lb loaded mag is not crazy. The 30 lbs listed is thus over-weight by 4 lbs, though perhaps that includes a scope, which can easily weigh 1-2 lbs for a 8-15x optic. So that’s not too badly off, really, though declaring the entire rifle as 25lbs loaded wouldn’t be crazy-talk either.

And that’s without even considering the fact that you now have a 15mm, 900gr projectile of appropriate aspect ratio to fling. All sorts of ammo is available for it.

So net/net, and unlike the pistols and to a slightly lesser extent, the SMGs and PDWs, the first entry in the rifles category makes some sort of internal sense, with the only thing being “off” about it is the range of the bullet. With a higher pressure, going up to the 62,000psi of the .300 Win Mag, and a bit more long-burning powder, getting to 17d out of this isn’t even unusual, and pushing to 6dx3 merely required a rifle that has a bit more powder burn and a 33.6″ barrel, which in a bullpup configuration is 12.5cm/5″ longer than assumed, or about 51″ . . . still 6″ shorter than the modernized M107 Barrett rifle with a 29″ barrel. The faster bullet gives it a longer max range, too.

So let’s assume that’s what we have here, and we get:

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 Anti-Materiel Rifle, 15x103mm 6dx3 pi+ 6+3 925/6,950 30/3.5 3 10 12B -6 4 $8,000/$200 2 0.20

Even with “ball” ammo against the equivalent of 10d armor, you’re still looking at 8d wounding, which is bordering on hamburger territory against people. Where 2d injury to the vitals is an instant kill, this will do such a wound through 16d worth of armor: DR56. With armor divisors of (2) or (3) and a large enough wound to prove horrible, vehicles armored with up to 2″ or so of RHA equivalent are no guarantee of safety. The rifle will also reach nearly four miles and still hit as hard as a sniper rifle at 100 yards. This is a credible threat even against TL9 powered combat armor, and the insanely overpowered HEMP will hit for 10d (5), which is almost enough for the DR 200 armors out there.

Almost.

Assault Carbine/Hunting Rifle, 7x35mmCL

Next up is a more pedestrian rifle, a 7mm projectile with WPS listed as 189 grains. That gives some room with which to work, and the 1/2D of 700yds is nearly attainable with a 140gr bullet with a 5-1 aspect ratio. In fact, I’m certain that with the crudity of my model, a properly shaped bullet will get exactly that. So that’s excellent.

Now, a rifle with a bulk of -4 is right up the alley of the Steyr AUG, the L85A1, and the G11 . . . all bullpup configurations with standard barrel lengths of 508 (Steyr) to 540mm (G11). Let’s use the G11 as our prototype, since the overall package is supposed to be futuristic, and we’ll say it sports a 540mm barrel.

The cartridge itself could be a 7x35mm caseless modeled after that of the G11 and still have about 50% more capacity than the 6.8x43mm SPC, so we’re not lacking for propellant power or stabilizer room here.

With a fairly conventional max pressure of 55,000psi, a reasonable duration of max pressure, and the stats listed, launching a conventionally-constructed bullet at 726m/s will provide the 6d damage listed in the book. The calculated 1/2D and Max ranges are 675/4,400yds, darn close to the printed values. The Acc 4 is probably low; a bullet of this aspect ratio paired with a rifle of this barrel length should not be less accurate than the M16, and should be at least as accurate as the REC7, which is a 6.8x43mm SPC with a 16″ barrel giving Acc 5 (Tactical Shooting, p. 64).

Weapon weight shouldn’t be too much different than the aforementioned 7 lbs of the REC7 empty; complexities in the caseless or combustible-case-telescoped (CCTA) ammo that would drive out weight can be assumed to be counteracted at least as much by materials improvements, and the old “Carbon 15” rifles by Bushmaster put an AR15 platform in a 4-lb rifle (empty). So a base weight of 6-7 lbs should be doable.

For this one, the RoF 12 should really be thought of as three or four high-cyclic bursts, with RoF of 30-35 for a 3-round burst, but fired in semi auto at 3-4 bursts (usually max 3). So from that perspective, a true RoF of 9, with mechanics such as treating Rcl as 1 for three rounds, then Rcl 2 or 3, then Rcl 1 again for three rounds, etc. There are extant rules for high-cyclic controlled bursts so you can choose those or make your own to taste.

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 Assault Rifle, 7x35mm CL 6d pi 5 675/4,400 7.5/1.5 12 50 9 -4 2 $1,600/$35 2 0.027

For the Hunting Rifle, an extra foot of barrel will in fact get you another point in damage and just shy of 200 yds of max range.

Against 10d flavor armor, you will definitely need a (2) armor divisor provided by some sort of heavy penetrating ammo, and if you need that, then a better version of the CCTAPX design used previously will likely get you something like 5d+2(2) pi+, which is about 3 points of injury per hit. That requires 2 rounds to hit KO to the vitals or even the brain. 6d is a great target for unarmored foes and is a serious round providing instant incapacitation or lethality in that case. Against more heavily armored foes, you’re going to need a more robust round.

Fortunately, with a bit more tech oomph (higher pressure, a bit larger chamber and thus more propellant) you can probably get back to the old standby of 7d, or a 140gr round at 2,750fps – congratulations, we’ve just rediscovered the .308. That extra die doesn’t seem like much, but it allows things like APEP, which will give 7d (3) pi-. Against DR 35 or 40, you’re looking at 24.5 rolled damage vs a net DR 12 to 14, for 5-6 injury per hit, or a decent wound on a non-vital location, and a horrid fatal one vs a vital location. APHC and APS will deliver 5-7 through armor, so are still quite ugly on a hit to the vitals. That extra die is pretty key here.

Gatling Carbine, 5.9x30mm CCTAPX

We established in prior work that the TL8 PDW styling – the 5.7x28mm – is unlikely to be satisfactory and so we built our own. We’ll keep with that concept, first refreshing the PDW design in our memory:

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 PDW, 5.9x30mm CCTA 3d+1 pi 3 500/3,350 4/1.0  20 60 9 -3 2  $1,650/50 2 0.022
9    with CCTAPX 3d-1 (2) pi+ 3 585/3,600 4/1.1 20 60 9 -3 2 $1,650/50 2 0.039

The new Gatling carbine has the same damage output stats and range as the pistol, so we’ll assume that the trick here is simply amping up the RoF in a heavier platform with more shots carried. With three barrels instead of only one, a 10-12 pound platform seems right, and increasing RoF from 20 to 50 or 60 isn’t crazed – the barrel assembly would need to rotate about 1200rpm to do this (20 times per second). That’s quick but not insane, though it is a lot of rotating mass. Let’s call it a 9 pound empty weapon. 200 shots would provide all of four seconds of continuous fire, but of course, our carbine would have RoF 9-12 and hold 50 shots, so that’s only 5 seconds as well. Carriage of 200-300 rounds isn’t nuts, then, as a goal. Let’s go 240, assuming four seconds at 60 rounds per second. The basic bullet would be 5.3 lbs just for the ammo, and probably calling it 5.5 lbs overall wouldn’t be insane, which makes the CCTAPX magazine weigh 9.6 lbs. That makes the loaded weight 14.5 lbs with conventional ammo, and 18.6 lbs with CCTAPX – in short, the reloads are way too light in the book.

That puts the weapon in the same class as the Browning BAR, but shorter. Stat line? More properly something like this:

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 Gatling Carbine, 5.9x30mm CCTA 3d+1 pi 3 500/3,350 14.5/5.5 60 240 10 -4 2 $2,400 1 0.022
9       5.9x30mm CCTAPX 3d-1(2) pi+ 3 585/3,600 18.6/9.6 60 240 10 -4 2 $2,400 1 0.039

Note that the cost of a full magazine of CCTAPX equals the cost of the weapon itself. That’s a lot of expensive tungsten and tungsten/copper being tossed downrange.

This one is an odd duck – a ridiculously high RoF weapon with only a few seconds of ammunition, it assumes that the RoF bonuses and spread are the key here. If infantrymen are on the order of fighter aircraft in speed and mobility, but kitted out in armor with DR 15 or so, then this becomes the equivalent of a fighter’s autocannon. Against armored troops, even the CCTAPX is going to be foiled by more than DR 20, but it’s more than a match for large groups of semi-armored foes. Very large groups, such as insectoid swarms where you assume that every single one of those 50-60 shots will eventually hit something.

Against human foes in DR closer to 35 than not, you’re counting on hitting something vital or a seam in the armor by filling the air with small (possibly very expensive) projectiles. This is a specialty weapon, to be sure.

To Be Continued

I’ve worn out my eyes for today, so I’ll pick up the final three entries later this week.

Overall, unlike the substantial redesigns required/desired around the pistols, SMGs, and PDWs, the rifles, with the exception of the ammo weight of the gatling carbine, aren’t inherently broken out of the gate.

That isn’t to say they’re optimized for the job, though. They’re enough for second-line troops or specialized threats, but each could be made a bit better or more suitable, especially against TL9 opponents, by optimizing the weapons for the technology a bit more. Higher pressures, larger chambers to give a bit more sustained power.

And of course, the velocity/damage boost you might get through a liquid propellant upgrade (which works out to about a 30% velocity boost) would help a lot here. Our 6d assault rifle at a fairly low 725m/s suddenly becomes 942m/s, which is in the 7mm Remington Magnum class, and 8d-1 does almost all of the things against high end TL8 body armor that we wanted out of our 6d round, plus allowing 3d shots, subsonic, for sniping against unarmored targets, and the subsonic round is born to be paired with integral suppression (I’m a big fan of this for noise reduction reasons anyway).

4 thoughts on “Sunday is GunDay: GURPS Ultra-Tech Rifles

  1. Good stuff as usual, Doug! You have reminded me (again) that I should probably publish those TL9ish weapons I’ve had in draft for some time now (and also my Gatling article)! May be something to do this week so I can stop looking at vehicular stuff for a bit!

    1. Thanks. We hosted a party yesterday that kept us hopping from about 1-9pm, so I wasn’t able to finish the post, nor do my own designs.

      That’s OK, I’ll get to them over the coming week or so. I’d forgotten how much fun this can be.

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