Gunday Continued: Ultra-Tech Rifles (Part 2)

In the last installment, a few of the basic weapons from the Rifles section of GURPS Ultra-Tech were examined. The basic premise was to see if the designed stats make sense, and evaluate them against an assumed mission statement: deliver 3d-6d injury after penetrating whatever armor the target is wearing. The typical foe was assumed to be a TL8 trooper wearing roughly DR 35/10d worth of hard/soft armor combination. This might be the TL8 Assault Vest + Trauma Plates (DR 12/5 + 23) or Advanced Body Armor (the generic form of DragonSkin, assumed to be as effective as the manufacturer claims).

Last time, we covered the basic Assault Rifle, and a derived hunting rifle firing the same ammo, the gatling carbine, and the 15mm payload rifle.

Today we finish up with some oddities, large-caliber specialty weapons that are either overdesigned or mis-applied in many cases.

Payload Rifle, 25mm

This weapon is basically a high velocity grenade launcher, similar to the XM25 CDTE with some performance enhancements in terms of magazine capacity and inherent accuracy.

The Payload Rifle is pretty clearly based on a hypothesized version of what was the OICW program, and evolved from a 20mm to a 25mm independent grenade launcher, which is covered in High-Tech in the 20mm version (before the 20mm grenade was shown to be underpowered). While notionally the 25mm has about double the explosive capacity of the 20mm round, in practice if the propellant doesn’t change that much, and the fusing doesn’t scale up with volume, you might get substantially more payload relative to the 20mm. Even with that, the assumed PLASTEX-B explosive used in Ultra-Tech is on the order of 2.5-3x more powerful than most conventional TL8 explosives. Net of that is that what this weapon can be modeled after is a fully proved version of the M25 “Punisher.”

The interesting thing about this one is that it makes extensive use of targeting software, and with an integral ballistic computer that after making a laser-ranging “shot” downloads all sorts of information into the software, including how far past the ranging location the shooter wants the explosion to occur. Then the computer just gives the operator a “put this dot on target” cue, and then so long as the shooter holds that point, it pretty much works, according to reports (always skepticism required) from testing of the prototypes in Afghanistan.

This probably means an aim action to lase the target (use Pyramid #3/77 “On Target” for this one), a Ready action to set the parameters, another Aim action (this time collecting a substantial targeting computer bonus), and then Attack until you want the aimpoint to change. Rapid Strikes (again, using On Target) can make some of these Aims and Attacks very fast.

One notable difference in the UT weapon is that its 25mm projectiles do a lot of kinetic energy damage – 10d worth. The weight per shot is listed as a full pound, or 7,000 grains, so this is no featherweight. The M25 system is supposedly a 14-lb weapon and the full combat load including 36 grenades is 35 lbs, yielding roughly 0.5 lbs per shot. Let’s go with the lower figure instead of the higher, and see if we can get the required 10d kinetic energy out of it. Some very, very tentative assumptions:

  • Use roughly shotgun pressures, of 20,000 psi, and a fairly low powder charge
  • Barrel is about 18″ long, bullpup configured
  • The projectile is 25mm and 113 grams (1750 grains), with an aspect ratio of about 2.5:1, which gives a density of about 5 g/cc, less than steel, much higher than water. Might be OK.
  • The case length is roughly 40mm long, about 1.5 calibers.

This gives a velocity on the order of a .45 ACP (about 250m/s), an 1/2 D range of 550yds, which is almost exactly what the “effective” range of the projectile is supposed to be, so let’s keep it. The delivered kinetic energy is about 3,650J, which is a bit less than that delivered by a .30-06, but with three times the momentum. It’ll kick, and feel like firing a .416 Barrett, and each shot is designed to be much more effective.

In order to get the 10d that is the assumed damage of the Ultra-Tech version, we’ve got some serious work to do, and we need to double the velocity of the thing. We will need to start by upping the firing pressure to pistol-level (about 40,000psi), and since the Bulk of the weapon is -6, we can assume a substantial barrel, more like the 730mm of the Anti-Material Rifle than the assumed 450mm of the H&K weapon currently in seemingly eternal development. That gets us to 7d+2, so still not enough. Need more powder.

OK, now we have a weapon that kicks like a shoulder-fired KPV ( and deliver just shy of 14 kJ compared to about 34kJ for the .50BMG) and will look a lot like the M95 AMR in terms of overall layout. It will fire a 113g projectile at just shy of 500m/s, still have a 550yd 1/2D range, but with a max range of 3,600 yds (about two miles) and deliver 10d worth of damage. The weapon description notes “a very effective muzzle brake,” and to that I say “no shit, Sherlock.” The Rcl is listed as the same as the Anti-Materiel Rifle, and that’s probably not wrong, given the assumptions. It’s got about 150% of the oomph of a .600NE in terms of momentum, but it also weighs about twice as much, so Rcl 6 is probably right, and “a very effective muzzle brake.”

Each magazine holds 8 rounds, and so weighs about 4 lbs for just the ammo, or about 4.5 lbs including the magazine itself (note the book lists 10 lbs, which is appropriate since I arbitrarily cut the projectile weight in half; turns out not so arbitrary, though, since that kind of projectile will possibly break bones when fired).

That leaves an empty weapon on the order of 33 lbs, which frankly is probably too heavy. Our 15mm AMR is a higher energy weapon by about double. Cutting down the base weapon to between the 15 lbs of the XM25 and the 26.5 lbs of the empty AMR, and get an empty weight of about 20 lbs, making about 24.5 lbs for the entire system.

That makes the basic stat line resemble this:

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 Payload Rifle, 25x35mm CL 10d pi++ 3+3 550/3,600 24.5/4.5 3 8 12B -6 4 $8,000/$200 1 0.50

Of course, the reason you light off a Payload Rifle is for the payload. What will roughly 1,250 grains (0.179 lbs) of PLASTEX B deliver? About 4d+1 in explosive damage. What does Ultra-Tech list? 4d cr ex [1d+1].

Hey. That ain’t bad.


Let’s check out some other ammo types.

  • Keeping a 1,750gr actual projectile (including 10% weight as a sabot) and making it tungsten or DU (both density of 19) gives a 12.5mm projectile with an aspect ratio of 5:1, which will hit for about 11d+2 (2) pi+ penetration. Velocity isn’t improved, really – this is just taking advantage of the native energy of the projectile. If we cut that down more – call it about a 7.33mm projectile – from the same energy content we get 14d (2) pi instead, with a cartridge that weighs 1,100 grains less (WPS drops to 0.35 lbs). Effective up to DR 98. Using APEP would increase the armor divisor to (3), and thus be good to about DR 147.
  • APHEX would be 10d (2) pi++ and linked 4d+1 cr ex [1d+1]. Effective up to DR 70.
  • Shaped charge would be 10d(10) cr inc + linked 4d cr ex [1d+1] using the HT rules, and 15d (10) cr inc + linked 2d cr ex [1d+1] with the UT ones. Effective up to DR 350.
  • SEFOP is 15d (3) cr inc, effective up to DR 158.
  • Thermobaric is 8d cr ex inc instead of 4d, doubling the explosive effects. Effective up to DR 28.

Only the APHEX and APDS style ammunition depend on the kinetic energy of the projectile, and they’re not the most effective. One way to go here would be to keep the weapon built to the high pressure standard, but for all of the explosive-based charges, use a lower pressure projectile that is much more forgiving on the shoulder, which would be Rcl 2 or Rcl 3 instead of 4 to 6, and only bring out the dedicated KE weapons when . . . well, in GURPS, I’m not sure when that would be better.

10mm Storm Carbine and Storm Rifle

The ostensible reason for this one’s existence is “when you need more punch than an assault carbine can provide.” Whether this requires a 10mm bullet is a good question, but let’s run with it. The WPS is listed as 0.04 lbs, or a total of 280 grains. That’s already going to be a problem, because a 10mm pistol bullet is going to be in the 180-200 grain range, and a properly shaped rifle bullet with 3.5-4.5 aspect ratio will be significantly heavier.

Additionally, the basic damage rating of the weapon is 7d pi++, compared to the 6d pi of the assault rifle in the basic stats, and the 7d pi that we get out of my modified stats in the prior article. That’s not really more punch, that’s a larger wound channel, delivered in a heavier weapon with lower RoF, the same Shots, and worse recoil.

At TL9, with the really cool projectiles really not available at 10mm, this weapon is really looking for a home, and not finding one.

I’m going to merge the Storm Carbine and Storm Rifle into one entry. The WPS of the 10mm CLR (caseless rifle) ammo is 0.06 instead of 0.04, which gives 420 grains to play with instead of 280 and thus making the Black Ops “Pinata Stick” more plausible. A properly-sized bullet (4:1 aspect ratio) with conventional mild steel or jacketed lead construction will be in the 340 grains category, leaving us 80 grains to work with. Is that enough? Barely.

We’ll use a 70mm long CCTA case, of 14.6mm in diameter. Unlike some of our prior attempts. that overall volume is going to make use of every bit of PLASTEX that can be stablized into it. We’ll scale the pressure based on the .338 Lapua, and assume a max pressure of 60,000 psi, and fling it out of a 540mm barrel, the same size as our G11-modeled assault carbine, though we could easily go higher, but probably not and keep Bulk to -4.

That gives us a nice 10d output, and it’s pi+, which means things like armor piercing and sabot rounds will be full piercing AND cut through armor well, so an APHC round might do 10d (2) pi, which will deliver 5d through DR 35, and very solidly meet our design goals for both injury and armor penetration. Range will go up as well, so

It’s a beast, though: nearly 7,900J of kinetic energy and a likely kick like a .458 Win Mag or .338 Lapua Magnum. This is an elk- or moose-killing rifle in an assault rifle package; basically the mother of all AR-10s.

This one should be able to count on Acc 5 easily, and a projectile this beefy will have a satisfyingly long range: 775yd 1/2D and 5,500 max. A sniper rifle version of this with a long barrel (call it 775mm) will deliver 11d, which isn’t much of an improvement but sometimes 1d is all you need, I guess – more importantly, the sight radius and accuracy will go up, maybe to Acc 6. The weight doesn’t seem wrong, and the weight per shot and loaded magazine weight would easily support 15 shots instead of 12. Legality class really should be in line with the Assault Carbine.

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC WPS
9 Storm Rifle, 10x70mm CLTA 10d pi+ 5+3 775/5,500 10/1.2 3 15 10 -4 3 $2,700/$35 2 0.060

Parting Shot

As I suspected, the 10mm rifles were problematic; at the levels of performance being discussed, there’s really no reason to amp up the caliber to 10mm from 7mm at TL9. AT TL10+, options for HEMP projectiles at 8d (5) show up, with an internal 1d-2 explosion (which I tend to treat as an auto vitals hit; that may even be official) and will result in enough internal damage to risk a KO even on a near-zero damage hit. More importantly, 10mm HEMP will deal handily with up to 40d worth of armor.

But that’s TL10, and we’re not there yet.

It occurs to me that for diffuse or non-living targets, such as swarms, there’s a benefit to the larger caliber, but again, for that sort of thing a bursting round is best. Big bullets vs undead or animated targets in a Monster Hunters or sci-fi campaign that have the functional equivalent of such is a real thing in GURPS, and so in that case, a 10mm assault carbine might make some sort of sense.

The payload rifle is a pretty impressive beast, but it doesn’t need to be that impressive, since the best possible effects can be had with much less kinetic energy via exploding projectiles. Allowing the basis for a high-KE round is nice as a fall-back, but a softer-launched primary loadout of SEFOP or HEDP with (say) a 5d kinetic capability  will likely serve overall better than the 10d kinetic projectile assumed.

The third part of this article will look at some purpose-designed rifles around some basic assumptions, likely making heavy use of liquid propellant technology, and even examining the still more impressive electrothermal-chemical weapons, which have about double the energy output (ETC) of a conventional round – though taming the muzzle flash will require some engineering.

8 thoughts on “Gunday Continued: Ultra-Tech Rifles (Part 2)

  1. I almost never assume anything other than HEAT (or later HEMP) in a 25mm and at TL10 or less nothing in UT lighter than one of those 30 ton light tanks will stop a 25mm HEAT. This does possibly justify the existence of those light tanks.

    I usually think of the 25mm grenade launcher and assume a guided round which makes the MAX the important stat rather than the 1/2D. The grenade launcher will give you enough range for many purposes but the payload rifle will actually look quite a bit like portable artillery.

    For the 10mm at TL9 I assume APEX and targets of robots/drones or light vehicles. IT: Unliving can really limit the performance of pure P type rounds.

    1. That further confirms the impetus to limit the KE of the round unless you really want max range.

  2. Having been in a game where we had access to both TL9 firepower (gadgeteer character) and were needing to go after Unliving targets (typically undead, sometimes light vehicles or combat bots) I will say that while it’s not nearly as much of a factor in real world, in GURPS games the big wound channels can be damn useful.

    Even in a non-supernatural TL9 game, combat robots are totally a thing that can show up.

    The real world version of that I’m assuming would be using an AMR for actual anti-material work, which I’m guessing is one reason they are known for such large bullets.

    Oh, semi-related. Are you planning on doing anything about the TL9 variant propellent options, like binary or electrothermal? I’ve always wondered how those would look with a more close look at them (even if the answer it ‘the books are a bit on the cinematic side about these).

    1. My design musings will touch on electrothermal-chemical and liquid propellant. I even reached out to David Pulver to make sure that my wanderings will be relevant to works that are in progress, wink wink.

      1. Really loving Gunday! Will you address the Electrothermal-Kinetic option in David’s Ultra-Tech Designer Notes? Do you consider it too much of a good thing?

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