Ultra-Tech Gun Damage Revisited

Yes, I'm still working on other things.  This won't become the Ultra-Tech Gun Blog (I leave that to better men). But yesterday's post created quite a stir, and I wanted to record some thoughts and some links.

First, this. Evidently not all of Pulver's errata made it into Ultra-Tech Revised, or at least the version I have, which is a shame.  But I find it more interesting what he says at the end:

Since the Weapon Design Rules, while functional, remain vaporware, some weapons in UT were not modified if deviated too much from Basic Set. --David Pulver

This doesn't tell you anything you don't know in your heart of hearts, I suspect, but it does confirm something important: there is no gun design system and there never was. He's using some baseline (likely from GURPS Vehicles) and then either fudging the rest, or just using stats from Basic, which are likely themselves converted from 3e (which probably used GURPS Vehicles).  So, in short, the design system is probably GURPS Vehicles with a large fudge factor. If you just make it up as you go, as long as things "feel" right, you're probably fine.

EDIT: Someone wanted to note that I'm misinterpreted this statement.  There does seem to be a design system, it's just not "publishable in its current form," and we're unlikely to see it.  My point about "you can probably fudge it because they do" still stands, however, given the final part of the statement.  I also doubt we'll ever see a behind-the-scenes look at the design system, as I get the sense SJGames is very much move moving away from revealing complexities that lurk behind GURPS design choices.

The second is this, which is probably the final word in most arguments about gun design in GURPS.

I've seen this article before and bounced off it a couple of times as incomprehensible. Seems I had to get to the point where I could invent it myself in order to understand it. --Skullman

Man, I know the feeling.  Well, we let's just dive in and see how it works out.

Parsing Gaming Ballistics

I find it best to start with a baseline that I try to work from. In this case, we'll go with the Heavy Pistol once again.  If we can replicate it in his system, we can see if we can replicate the rest of our guns and if we can do that, we have a pretty good grasp of what's going on.

Icky Equations: Armor Penetration
 
Let's start with penetration of armor.  If we first find the square area via the equation he offers us for bore size, and then input the numbers until we get our 10 damage, we come to about 710 kj of energy for 3d damage with a 10mm round.  How well does this track with "reality?" Well, a 9mm parabellum round has about 600kj, and if we put this into the same formulas, we come out with 9.6 damage, which rounds up to 2d+3, which is high compared to what GURPS says it should be.

Armor Piercing Rounds

We can ignore this and replace it with the values from UT rounds, which are more-or-less the same.  AP is close to a tungsten carbide core, and APEP is close to a DU round.

Icky Equations: Wound Channel and Blow Through

Taking the above formula to its logical extreme, we'd all be firing hypervelocity DU needles.

>.> @ UT's Gauss Rounds

Anyway, this seems mostly a relic of the old, 3e GURPS, because 4e accounts for a difference between armor penetration (raw damage) and actual wounding (the damage modifier).  We need to know the weight of the bullet in kilograms, which we can derive from the 10mmCLP WPS for ultra tech and, based on the suggestion in the article, use 2/3rds of it for the actual bullet weight.  Then, we need to knows the speed of the bullet.  We expect about 15 damage, which means our bullet needs a blistering speed of 1735 meters per second.  If we assume the bullet is heavier (all bullet, no powder, somehow), we can drop the speed down to 1150 mps.

Does this compare with real-world stats? Well, our 9mm round is 8.04 grams and flies at about 400 m/s, which gives us about 5 damage.  Taken together, the 9mm looks more like 3d damage with pi- than 2d+2 with pi.  I don't know what to make of that.

Interior Ballistic, Revisited

Ahh, finally an equation to give me nightmares. With some trouble, I was able to work out some essential values here.  I used as my basis the 9mm Beretta, and a lot of this frankly turns into guess work.  I assume the chamber is the size the bullet (Which I found given as 19mm, so 19mm times the surface area of the 9mm bullet).  For the length of the barrel, I found that on wikipedia, and did similar math, and then did a bunch of research to figure out how to do a natural logarithm on a spreadsheet and I came up with the final results.  I knew the expectation was ~600 kj, and this happens... if we assume a maximum PSI of 30,000, which is the low end for modern pistols.

What about our 10mm heavy pistol?  Well, if we just increase the diameter to 10mm, assuming all other things are the same, we get to 3d immediately, even at the same lower interior pressure. If we allow the pressure to rise the maximum suggested value of 40,000 PSI (representing superior technology), I get ~1000 KJ for the bullet and that gives us a damage of about 3d+2, which suggests that the original value I had for the Heavy Pistol having a shorter barrel might not be all that crazy.  Alternately, I've read that a pistol width-to-length ratio of 1.5 is more typical, with the 9mm closer to 2:1.  If we do this, we get 900 KJ at PSI 40,000, which gives us 3d+1 damage.

Regardless, I feel like this is excessively finicky. How often will I want to design the exact dimensions of my gun?  It *could* be done, and you could really see what happens when I expand out my 10mm round to fit in a carbine-length barrel, but to get a sense of how these *actually* worked would require knowing a great deal about my weapon, and I'd almost rather just know the KJ are.

On the other hand, we can get a sense of what some barrel lengths might be and, if we know that, all the rest of the values just happen automatically.

Based on looking up barrel lengths for various weapons:

• Snub Pistol: ~50-90mm (Typically 75mm)
• Pistol: 100-125 mm
• Submachine Gun: ~150mm
• Carbine: 200mm to 500mm (probably more towards 200mm)
• Assault Rifle: 400mm
• Shotgun: ~500mm
• Rifle: 500mm
• Sniper Rifle: 600mm

Revising Ultra-Tech

 If we use these values as a basis for a new set of guns inspired by HT, we get the following damage values.  Pistols assume bullet ratio of 1.5 and PSI of 40,000; rifles assume bullet ratio of 3 and PSI of 560,000.

Heavy Pistol: 10mm pistol with 100mm barrel: 3d+1 pi+
Holdout Pistol: 7.5mm Pistol with 75mm barrel: 2d pi
Magnum Pistol: 15mm Pistol with 125mm barrel: 5d pi+
Medium Pistol: 7.5mm Pistol with 100mm barrel: 2d+1 pi

Machine Pistol: 10mm with 100mm barrel: 3d+1 pi+
PDW: 5.7mm with 150mm barrel:  2d pi-

Anti-Materiel Rifle: 15mm with 600mm barrel: 12d-1 pi+
Assault Carbine: 7mm with 400mm barrel: 5d-1 pi
Hunting Rifle: 7mm with 500mm barrel: 5d piStorm Carbine: 10mm with 400mm barrel: 6d+2 pi+
Storm Rifle: 10mm with 500mm barrel: 7d pi+

The Payload rifle will use 15,000 PSI, comparable to a shotgun and has a 600mm barrel: 9d pi++

It's interesting to me that the rifles have so much less damage with these numbers than they do in Ultra-Tech.  In fact, these aren't much better than what we get in HT.  Perhaps the guns in UT really do do too much damage, or perhaps I've screwed up some numbers somewhere.

I'm not sure I need this level of detail or these numbers, and they don't actually resolve some of the more pressing questions I have, such as the weight of the weapon and its range, but it's been an interesting exercise and I hope people found it useful.