Ultra-Tech Rifle: From the ground up

Over the past, in various GunDay posts, I’ve taken a look at the various Ultra-Tech weapons with a mind to both determining the feasibility of the weapons, and also checking their performance.

One of the more interesting posts, to me at least, was a ground-up redesign of a service pistol. The “ideal” pistol seemed to fire a 5.9mm projectile of unique engineered construction, which basically fired an advanced projectile that combined the AP capabilities of a tungsten tip along with a base that the tip gets pushed in to, which expands like the petals of the Ranger SXT modern-day JHP in order to enlarge the wound channel of the projectile to greater than 10mm, the threshold for pi+.

That gave us a projectile that wound up doing 2d (2) pi+, which wounds like a .45ACP and penetrates like a carbine, with a 20-round capacity that disappeared into a standard pistol grip.

Along the way, I also took a look at the Liquid Propellant (LP), Electrothermal-Chemical (ETC), and Electrothermal-Kinetic versions of various rifles, and found out (honestly, to my surprise) that all three were feasible with the right assumptions.

This particular article will take from all of those technologies in order to come up with a competitive TL9 rifle that’s an actual upgrade over TL8, and puts the right amount of power into the hands of the standard infantryman.

Rifle Requirements and Technology Selection

We’ve talked about this before, but it’s worth repeating. The end product needs to deliver the right wound, at a reasonable distance, and hold a suitable number of shots. Recoil should be low, maintainability high. The manual of arms should not be unduly complex. If possible, since we are designing for GURPS after all, we should take advantage of breakpoints.

So, there’s injury and there’s penetration. We’d like to ensure we can try and defeat, on the average, the typical TL8 combat armor. That’s a tall order at times, but if our “standard” ammo comes with a (2) armor divisor, we can assume that threat is about 5d worth of penetration absorbed. If one wants to deliver a potentially fatal wound to a non-vital location, the pi+ that we’ll assume the CCTAPX ammunition provides requires 4d residual penetration, which will increase to 6d injury (enough to, on the average, hit for 21 HP of injury, taking Joe Average from HP to -HP in one shot). So 9d (2) pi+ will face down DR 35 (10d worth of ammo), and deliver 4d pi+ per hit.

In order to get to pi+, we’ll want the basic projectile to be at least as large as the 5.9mm projectile in the pistol-size version of the CCTAPX, but we need not constrain ourselves there. I don’t know that we need to push the caliber all the way up to the 6.5-7mm caliber that represents the usual compromise between full-power 7.62mm rifles and the typical 5.4-5.8mm infantry rifle of TL8.

Still, one might not always want to fire bullets that will wind up costing $1-2 per shot, so having a viable ball round that’s your usual jacketed lead isn’t a bad plan. So we’ll punch the CCTAPX notional diameter to 6.5mm.

To reach out and touch someone, we’ll want to have a good 1/2D range. The composition of the round – a tungsten tip for penetration and a tungsten-copper base for machineability, ductility, and mass – will take care of density, and if we push the overall projectile aspect ratio to 5:1 (my favorite for sectional density and therefore reach), we should be able to get a satisfactorily high 1/2D range, and very satisfying maximum range.

The overall projectile will be non-trivally long: 32.5mm, or a bit more than an inch and a quarter. But since the overall length of the Mk318 Mod 0 (the new USMC 5.56x45mm cartridge) is 2.2” long (just under 56mm), this represents a more svelte magazine from front to back. In fact, the length compares favorably to that of the 10mm Auto pistol.

But the cartridge, you say! The projectile is all well and good, but it’s the entire package that feels the need to feed. Well, I’m going to make the second tech-upgrade choice here, and select Liquid Propellant from the list of alternate technologies. I very much liked the idea from my prior article of a firing chamber that was basically a rounded cylinder that provides such a large combustion volume that the expansion down the barrel is negligible. The pistol version suggested a 30mm cylinder (about 1.2”) for a 5.9mm projectile, so we’ll use the same ratio here, and go with a combustion chamber of 33mm diameter and 65mm long. This should be very healthy.

A rotary chamber like the G11 should allow for high-speed feeding without even necessarily having to disturb the pressure vessel – it’ll be a top-mount magazine, four projectiles wide (about an inch), 200mm long or so. That will hold 120 projectiles per reload. I’ll assume a bottle filled with binary propellant is an extra, and work out the details there in a bit.

For weapon length, let’s see what we can eke out of a 30” (750mm) weapon. The chamber is 65mm long, and if we overdesign the hell out of the chamber walls, we can probably assume 85mm eaten for the combustion chamber, plus another 35mm for the projectile in the rotary projectile feed. 120mm out of 750mm thus eaten up with mechanics, giving a 630mm barrel potentially remaining. That’s nearly 25”, which is longer than the old-style battle rifle barrels, and this combination probably represents about the most acceleration you can push through such a short rifle. Actually, let’s knock that back by a bit to mount an integral suppressor, which we’ll assume that consumes a few inches of barrel as well. We’ll assume a 23” effective barrel length.

The 6.5mm expanding projectile – and since the CC in “CCTAPX” is combustible-case telescoped, the proper designation here is probably just APX – is comprised of a pure tungsten tip that occupies the top 1/3 of the bullet volume, and the base is the remaining 2/3. The volume itself is about 750 cubic millimeters, which gives 250 for the pure tungsten tip and 500 for the base.

That base is a density of 14g/cc, and the tip is 19g/cc, or .216 and 0.293 grains per cubic millimeter. Yeah, yeah, horrible mixing of units. Still, that means the tip is .293 * 250 = 73 grains and the base is 108 grains – let’s call it a total of 180 grains, with 70 on top and 110 on the bottom, just for fun. Again, note that 70gr for just the AP tip is closer to the AP M995 5.56mm projectile, and heavier than the M193 (55gr), and M855 (62gr) projectiles. The typical 6.8x43mm SPC throws a 110-115gr projectile, which is as heavy as the base of this bad boy. So we’re launching a much heavier projectile in our 6.5mm package.

Given all the other inputs, if we want 9d out of the package, we only need to charge the combustion chamber to 50,000psi. That’s not a low pressure, but it’s not obscene, either.

There’s no question that this package will deliver at least 5d+1 (2) pi, because even if you give zero impact to the 110gr following body that’s what the penetration would rate as. The follow-on body itself rates at a non-trivial 7d pi+ even without the controlled expansion.

So there are a few ways to look at this one. One is a duplex type round, where the initial projectile hits at 5d+1 (2) pi, and if that penetrates armor, you hit for 7d pi+ more, which is enough to thoroughly frak up nearly any humanoid target, as it will rate about 36 injury on the average. That’s in a way uglier than the design target of 9d (2) pi+, since if it hits 10d armor, it’ll punch through and still hit for 7d pi+; the target design will punch through and do 4d pi+.

Calling it 9d (2) pi+ as a single-stage, is thus a degradation in wounding potential, though it might overstate the AP potential (using the same ratio of about 75% penetration of the full thing, it should be closer to 7d (2) penetration, but the vast expansion of the base would provide more like 5d pi++ or 7d pi+ by itself . . . I’m tempted to compromise and call it 8d (2) pi++, which given the much higher additional mass of the projectile portion that expands isn’t unreasonable.

The Rifle

So ultimately, what we’re left with here is a 30” rifle, the last 3” of which represents some bleed space to pull noise and muzzle flash off of the 23” barrel (584mm).

Each magazine will hold 120 projectiles, which by themselves will mass about 3.1 pounds (!). Calling the entire magazine 3.25 pounds is thus not far wrong. It’ll look a lot like a P90 or G11 magazine, as well – maybe 35mm tall, perhaps 30mm wide, and 225-250mm long, including springs and other mechanisms.

If you want the projectile magazines to be “drop-free,” it might take some doing, with the grip a bit forward to accommodate the magazine. Still, putting that grip 12-15” from the shoulder will accommodate an underslung magazine with inches to spare, and allow gravity to strip a spent mag. If you put it on top like the P90 or G11, you have even more room.

For base rifle weight, we’re going to need to offset the ammunition weight with a very lightweight construction, and assuming we can make the thing weigh about the same as the 4 lb weight of the Carbon 15 rifle (with 16” barrel) might be pushing it – a 24” barrel for a 6.5 Grendel is 3.4 lbs by itself. Still, if we say that the entire rifle sans ammo is 5 lbs, we can put a lightweight optic on there, throw in a 3.25-lb magazine, and have the entire package come in at 8.25 lbs, hit for 8d (2) pi++ or thereabouts, and have more ready rounds in two magazines than the basic US combat load has in seven.

The 1/2D range of this weapon will be 1,050 yds, with a Max of 6,975.

If you don’t need to face advanced body armor, you can just use a solid brass projectile at 100gr, and get 9d pi out of it with a 1/2D range of 540yds, Max 4,750yds. It will be vastly cheaper, and the magazine weight will be 2 lbs each.

No changes other than swapping out the ammo are required to “upgrade” from 9d pi to 8d (2) pi++. That military-grade load is a threat with a shot to the vitals at up to DR 42 or thereabouts. One can also dial the load down, of course, and subsonic, with a velocity of 330m/s, will hit for 3d (2) pi+ (3d is valid from the overall velocity and mass; stepping down pi++ to pi+ also seems legit; nothing changes the tungsten core) and be fully suppressed.           1/2D range doesn’t change (that’s a property of the bullet), max drops to 4,325yds. That will also get you about 8x the shots in the reservoir – the pressure requirement for this reduced velocity is very, very low.

I would expect the base Acc with this barrel to be 5 or even 6; let’s call it 5 to account for mass production, but with a 23” barrel getting Acc 6 for Designated Marksman versions of the weapon simply through tolerancing would be fairly trivial.

Standard optic for this would be a 3-4x magnifier staged in front of a collimated sight for the line rifle, and having a 1-10x variable magnifier in front of the same red-dot sight would allow both eyes-open shooting at close range, and taking advantage of the 1,000yd 1/2D range of the military 6.5mm APX load as the situation requires.

There’s no reason not to make the weapon like the G11 in its ability to fire controlled bursts, either. So the RoF is 9 when uses in full-auto mode, but also can fire three bursts of 3 rounds with Rcl 1 within each burst, but Rcl 3 between bursts or for single shots. So make the roll exactly to by 2, and you hit 1, 2, or 3 times. Then if you make it by 5, 6, 7, you hit an additional 1, 2, or 3 times. Then finally if you make it by 10, 11, 12, you hit with all nine shots.

TL Weapon Damage Acc Range Weight RoF Shots ST Bulk Rcl Cost LC
9 6.5mm APX Rifle 8d (2) pi++ 5 1,000/6,975 8.25/3.25 9/30 120+1(3) 10 -4 3/1 $1,750 2

5 thoughts on “Ultra-Tech Rifle: From the ground up

    1. Thank you for the compliment. Groups as a role playing game system does not require this soda danger did you can just write down stats and move along. However, I did want to see what a series of reasonable assumptions might do in terms of using the model that I built to project the high-end performance for a next technology level liquid propellant combat rifle

Leave a Reply

Your email address will not be published. Required fields are marked *