[HT] Gun Design and the 5.56x45

[Seneschal]

Lately, I've been trying to frankenstein something like the Vehicles 3E gun design system, with parts from Pyramid 37: Blaster and Laser Design. Now, unless my search-fu is truly rotten, there has been no official 4E weapons design system of anything similar and, while I'm not exactly in the loop, I believe nothing like that has been announced. I thought I'd give it a shot, and quickly realized it's one heck of a task, but I've been able to put together... something. For the purpose of my campaign, the resolution is fine, but it's faaaar from comprehensive.

While putting together formulas for Damage, Range, Weight, etc., I've been consulting Douglas Cole's ballistics spreadsheet to see if our numbers match up (at least roughly), and I've come across something strange - both the Vehicles 3E and the ballistics spreadsheet agree that a 5.56x45mm NATO cartridge should have 1/2D Range of about 400 yards, but this conflicts with every other TL8 rifle described in High-Tech or anywhere else. Seeing as I'm not much of a gun aficionado, I may have missed something, but this seems like a big discrepancy between design systems.

These are the numbers I punched into the ballistics spreadsheet, and what came out:

Chamber Pressure: 62,000 psi
Barrel Bore: 5.7 mm
Case Length: 44.7 mm
Chamber Bore: 9 mm
Barrel Length: 370 mm
Bullet Mass: 63 grains
Aspect Ratio: 4
Burn Length 19.556 mm
Projectile Caliber: 5.7
-------------------------
Velocity: 933 m/s (3061 fps)
Kinetic Energy: 1777 J
Damage: 5d+1
Tumble Range: 288 yds
1/2D Range: 435 yds
Max Range: 3531 yds

Is any of that wrong? I got the numbers off of Wiki, and I only have a tentative grasp on what each of them does, but it seems about right. Now, here's the Vehicles 3E formula:

1/2D = sqrt(5.56) (bore size) × 250 (below 6mm) × 1 (conventional gun) × 0.5 (medium barrel) × 1 (rifle powder load) × 1 (TL8) = 294.7 yards

Oops, my mistake - it's even worse! A mere 300 yards! And while 3E maybe scales that stat differently (I wouldn't know, it's my GM's book; I've only ever played 4E), that wouldn't explain why it models certain other rounds more accurately - 7.62x51 NATO gets Range 790.

On the other hand, most GURPS equipment books list 700-800 yards as 1/2D Range for the 5.56. Which one is more accurate? What causes the discrepancy anyway?

Incidentally, this is just one of the hiccups I've come across while trying to cobble together this thing. I'm starting to realize why it has remained in development hell for so long, and why the individual book designers prefer to go by rule of thumb. It just bugs me that HT and UT stats don't match up, and that whenever I try to build something that fits into the established rules, I have to reverse-engineer the author's thought process by staring at the weapon tables.