[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.

[Purple Haze]

In the real world 300 yards is just about right for when it is down to half damage.

[Seneschal]

Originally Posted by Purple Haze

In the real world 300 yards is just about right for when it is down to half damage.

Which makes sense, as it barely has enough velocity to tumble right out the muzzle (in an M4 carbine), let alone 700 yards down-range.

However, why is the official 1/2D range more than twice that? What formulas are the authors using?

[DouglasCole]

I can't vouch for the accuracy of the table, but

www.hpbt.org/articles/223.doc

shows that the velocity of the bullet (62 g from 20" barrel) falls to half velocity at . . . about 600 yards.

[Anthony]

Half damage range, in GURPS, is rarely actually the range at which a round's damage would be halved; it seems to come closer to 'maximum effective range'.

[Sam Cade]

Originally Posted by Seneschal

Which makes sense, as it barely has enough velocity to tumble right out the muzzle (in an M4 carbine), let alone 700 yards down-range.

Tumble is a bit much. Yaw is more accurate.
Yaw is a function of bullet design not velocity. All spitzer shaped bullets yaw after impact since they have insufficient rotational velocity to maintain their nose forward orientation in the much denser medium.

A 55gr 5.56 M193 will reliably fragment if its impact velocity is greater than 2700fps. As the bullet begins to yaw it is torn apart. At lesser velocities the forces are such that the bullet stays in one piece and creates a "J" shaped wound as it swaps ends. M193 from a M4 carbine drops below fragmentation velocity at around 100 yards and is at half muzzle velocity at around 500.

[Seneschal]

Originally Posted by DouglasCole

I can't vouch for the accuracy of the table, but

www.hpbt.org/articles/223.doc

shows that the velocity of the bullet (62 g from 20" barrel) falls to half velocity at . . . about 600 yards.

But but... your spreadsheet! It lied to meeee.

I just tried it out, even with a 508 mm barrel, the range (420 m) stays the same, though the damage and tumbling range did increase. And I think I've seen you write around the forums over the years that 1/2D range only depends on the cartridge.

I mean, if I'm to get Range 750/2,900 (of the M4A1 in High Tech) using the Range formulas in Vehicles, I have to almost triple the range (and somehow exclude other cartridges for whom the formula works just fine). ;___;

[Ulzgoroth]

Originally Posted by Seneschal

However, why is the official 1/2D range more than twice that? What formulas are the authors using?

I've seen it said that the official 1/2D values don't really have anything to do with what they purport to.

[DouglasCole]

Originally Posted by Seneschal

But but... your spreadsheet! It lied to meeee.

I just tried it out, even with a 508 mm barrel, the range (420 m) stays the same, though the damage and tumbling range did increase. And I think I've seen you write around the forums over the years that 1/2D range only depends on the cartridge.

I mean, if I'm to get Range 750/2,900 (of the M4A1 in High Tech) using the Range formulas in Vehicles, I have to almost triple the range (and somehow exclude other cartridges for whom the formula works just fine). ;___;

The half-damage formula in the sheet uses an exponential decay in velocity based on, I recall, mostly sectional density (mass divided by frontal area). It's not going to be generically accurate without resorting to an input value of ballistic coefficient or a calculation of such a priori.

Were I do to it over again, I'd probably try to best-fit BCs from gross parameters like length to diameter ratio, sectional density, and "quality." Maybe an ammo design page like I did for bows in The Deadly Spring. But I'm not that interested in revisiting the work. There's a design system coming, doncha know . . .

[DouglasCole]

Originally Posted by Ulzgoroth

I've seen it said that the official 1/2D values don't really have anything to do with what they purport to.

True, though I think in 3e at least, 1/2D did double duty as a half-damage but also when you lost the benefit of Acc. So it was a blended function.

4e is better that way, especially with the limits on total effective skill introduced in Tactical Shooting. But the values themselves were not revisited, as I recall.