GURPS Ballistics – The 1/2D Range

One of the features of Third Edition (and sorry, I never played Man to Man, The Fantasy Trip, or GURPS First or Second editions; my first experience was with Third Edition, maybe even 3ed Revised) that was retained as-is into Fourth Edition is that of the half-damage range, commonly abbreviated 1/2D.

In third edition, after you passed the 1/2D range, not only did you lose half your damage (the ‘what it says on the tin’ part), but you also lost the benefits of accuracy (or lost half of it, something like that). So beyond a certain range, you were only using your pure skill, and got no Aim bonuses.

In Fourth Edition, they did away with the Acc part (and good riddance), but not only retained the concept of 1/2D, the numbers were retained as well.

Does that work?

Well, absolutely it works on one level. GURPS penetration/damage numbers are based on the square root of Kinetic Energy, which means that for penetration, the 1/2D number is actually the point where the velocity of the bullet has fallen by half.

OK, fair enough. And certainly a better design choice than having many damage increments or something else more complicated.

What about the values?

Let’s take a look at some common cartridges, and peruse the High-Tech Weapons Tables.

.22 LR: 70 yds
.357M: 190 yds
9mm: 160 yds
.40S&W: 160 yds
.45 ACP:  150 yds
7.62x39mm (AK-47): 500 yds
5.56x45mm (M16): 800 yds
7.62x51mm (.308): 1,000 yds
.30-06: 1,100 yds
.50 BMG: 1700 yds
12G Shotgun: 40 yds

So, notionally, each of these hits half its velocity at these ranges. Right?

Let’s see what my spreadsheet (no true ballistics calculator, true) says. It’s been close to right before, so:

.22 LR: 226 yds
.357M: 257 yds
9mm: 260 yds
.40S&W: 310 yds
.45 ACP:  290 yds
7.62x39mm (AK-47): 435 yds
5.56x45mm (M16): 370 yds
7.62x51mm (.308): 645 yds
.30-06: 645 yds
.50 BMG: 940 yds
12G Shotgun (shot): 120 yds
12G Shotgun (slug): 225 yds

So, how did we do? Not well – though if people have access to real ballistics data on these bullets, it would be an interesting check on my own numbers! Overall, pistols retain velocity about 100 yds farther than GURPS would have them. For rifles, with the exception of the AK-47 bullet, it seems like “too high by a factor of two” is closer. Interesting!

I’d like to check my shotgun numbers with real data. I’m positive that my model is OK for supersonic rifle-shaped projectiles, and much less good for subsonic ones. For example, my understanding is that once you go subsonic, you lose velocity relatively slowly, accounting for the increased deadliness of projectiles out to max range, which my sheet does not model well, but GURPS probably does, since you only get to half-D once.

Parting Shot

This is an Nth degree quibble in the land of a game where quibbling is mostly all you can do. Does it bother me? Not really, but a comment by +Justin Aquino about using the 1/2D range as a proxy for velocity got me thinking about 1/2D, and I got to thinking about it a little more. The numbers my sheet produces I think help with certain levels of plausibility, if you’re shooting pistols at moderate distances or rifles at long ones.

Also note that the 1/2D range is basically a function (to 1st order) of the shape and weight of the bullet, the primary impact being sectional density, and ballistic coefficient/shape being another (though sectional density is going to be a big input to BC). Long, skinny, well-shaped bullets like the 6.5 Grendel will retain their velocity very well, which is why that cartridge basically has the same 1/2D range in my model (640 yds) as the 7.62x51mm and .30-06 (and those two fire essentially the same bullet). That’s not right – actually Alexander Arms shows the 120gr 6.5 Grendel with a 1/2D of over 800 yards (!), so shaping can be a Big Deal – but it does give a feel for the numbers (and that my own figures probably are off by as much as 30% as well!).

Ultimately, most games will not turn on whether the 1/2D of a bullet is off. In fact, most RPG scenarios are probably resolved within easy pistol range – less than 30 yards. But for those that aren’t, and one memorable Black Ops game featured an exchange of fire at well over 1,000 yards distance – it can be important.

5 thoughts on “GURPS Ballistics – The 1/2D Range

  1. You could simplify all of these and get "close enough" if you said that pistols have a universal 1/2D of 250 yds, most rifles a 1/2D of 500 yds, and heavier guns like a .50 cal (actually, that's the only representative) have a 1/2D of 1000 yds. Shotguns would, I guess, be 125 yds for shot, 250 yds for slug. That would have the dual effect of making the numbers closer to your model, and also maybe a bit easier to use in play.

  2. Don't know how helpful it is but Hornady do a ballistics calculator that goes to 2,000 yards and Winchester do one that give the stats you need to put into the Hornady (who are meant to have them but I can't find it:D)

  3. "sorry, I never played Man to Man, The Fantasy Trip, or GURPS First or Second editions; my first experience was with Third Edition, maybe even 3ed Revised"

    :shakes head in stunned dismay: You lead a deprived existence, man. But I guess not everyone could get in on the ground floor….

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