The GURPS rules for hollow point or expanding bullets in GURPS are pretty simple: you pick up one level of increased wounding modifier in exchange for having an armor divisor of (0.5), meaning that your projectile is really quite poor at penetrating armor, but pretty good at injuring people.

So 2d+1 (0.5) pi- would normally average 8 points of penetration through armor, and 4 points of injury on an unarmored person. Against DR 4, on the average it would be blocked. Why one would fire off a pi- hollow-point is beyond me, of course – the example just shows how the rules work.

A 9mm FMJ would be 2d+2 pi normally, and thus do 9 points of armor penetration, 9 points of injury, and 5 injury through DR 4. A 9mm hollowpoint would (in RAW GURPS) would punch through only DR 4.5 on the average, and do 13.5 points of injury against an unarmed person. Only one point (on the average) would punch through DR 4, resulting in 1.5 injury.

A .45 ACP would normally do 2d pi+, and in hollowpoint will do 2d (0.5) pi++. So again, against an unarmored person, 7 points of penetration and 10.5 of inury in ball, and 14 in hollow point. Through DR 4, 7 won’t penetrate in JHP, but ball will do 4.5 injury.

In any case, the RAW is simple.

The Ballistic Calculator

My ballistics calculator handles things a bit differently. It recalculates penetration based on the expanded diameter of the projectile instead of applying arbitrary modifiers. This can produce truly monstrous wound channel modifier (though GURPS caps at pi++), but also gives a nice, straightforward damage reduction that isn’t quite as harsh as the (0.5) armor divisor.

Now, let’s look at three examples. A 1.3 expansion (typical of some .45ACP), a 2.0 expansion (which is typical of Jacketed Soft Point rifle bullets), and 1.65 (which works well for modern 9mm and .40 S&W that was engineered for large expansion).

• A .45 ACP with no expansion has 2d with a 1.5 modifer. (7 points)
• A .45 ACP wth a 1.3 expansion has 1d+3 with a 2.1 modifer (6.5 points, or )
• A .45 ACP with a 1.65 expansion has 1d+2 with a 2.7 modifier (5.5 points, or -22%)
• A .45 ACP with a 2.0 expansion also does 1d+2 with a 3.4 modifier

A 6.8x43mm would do 6d with a 0.7 modifier unexpanded

• The 6.8 with a 1.3 expansion would do 5d+2 with a 0.9 (21 to 19.5, or -7%)
• The 6.8 with a 1.65 expansion would do 5d with a 1.2 (21 to 17.5, or -17%)
• The 6.8 with a 2.0 expansion would do 4d+3 with a 1.5 (21 to 17, or -19%)
So between soft and expanded, you’re looking at about -20% for effective hollow-points. That’s about -1.5 per 2d damage, regardless of caliber. If you want to account for soft bullets, it’s more like a (0.8) than (0.5).
So a 9mm would go to 2d pi+, and a 6.8mm would be 6d-4.5, which is 5d-1 pi+.
Against armor, it will tend to do a bit better than the RAW, but still worse than unarmored flesh.
The (0.8) will be annoying in play; I’d tend to ditch it, but if you can quickly just increase DR by 25% before figuring penetration, you’re good. 5d-1 pi+ carbine bullet vs DR 12? It becomes DR 15, and likely on the average will do 1.5 penetration, increasing to a bit over 2 points of injury on the average. By RAW, it’s 6d (0.5) pi+, and DR 12 goes to DR 24, and will almost always bounce.

Parting Shot

This is for extreme realism fans, and even so, the modeling work required to pull this off is probably more than most people need. But the RAW really nerfs the penetration abilities of JHP bullets, making them devastating against
unarmored targets, but utterly useless against any sort of armor. That’s true to an extent, but not as much as GURPS makes it.

The trade-off is a lower DR increase relative to RAW, but also just slightly lower raw penetration (which takes the edge off the larger wound modifiers when not using the granular, limited nature of the pi-/pi/pi+/pi++ scale.
For games, the RAW is, without a doubt, easier. The math-heavy version makes for better comparisons where The Reloading Press is concerned.

1. "So 2d+1 pi- would normally average 8 points of penetration through armor, and 4 points of injury on an unarmored person. Against DR 4, on the average it would be blocked."

No, 4 still penetrates and you get 2 points of damage from it; on average.

1. My train of thought must have inserted a (0.5) armor divisor in there, but for no good reason. Fixed.

2. No worries. I once had two combat sessions where I took the DR1 from normal clothing with (0.5) and doubled it to DR2…

2. You calculation of averages is misleading.
"So 2d+1 (0.5) pi- would normally average 8 points of penetration through armor, and 4 points of injury on an unarmored person. Against DR 4, on the average it would be blocked."

It is indeed completely blocked often than not, but sometimes it will get through. If my math is right average damage is 1.0 for HP and 1.8 for std in this case.

1. Not misleading – it reflects the rules I use, which include armor as dice. That would, indeed, simply stop the projectile. I don't doubt that your math of "no penetration" and "some penetration" averages out as you say. Unstated assumption on my part, but a more realistic one in the case of firearm penetration.

2. I guess you could say that it does 3-13 with a chance of 0-5 penetrating and doing 1-2…

But shouldn't it be 2d+1(0.5) pi if it started as 2d+1 pi-?

3. I just strung some numbers together, rather than trying to model a particular bullet. I see I did more confusing than explaining.

4. When I find that I've been babbling instead of narrating, I just yell, "ROLL DODGE!" and the players forget I was momentarily insane.

3. Wonder what it'd be like to have normal penetrating damage, multiplied DR (with a x1.5 version for 'tweener' rounds like JHP) and 'damage rolled after armor if penetrated' and thus dispense with the wound modifiers. It would have the odd effect of mega wounds upon 1 point getting through, though, unless fiddling more with it (i.e. 'can do no more damage than double penetrating damage').

So for example 6d with x1.5 just has '3d after armor' and multiplies armor by x1.5 rounded up. If no armor just roll 9d. All the math is pre-calc'd, mostly (odd multipliers are no differently calc'd than High-Tech and easy). Technically you can pre-calc most armor by writing it with 'DR vs. HP' or somesuch.

But maybe I'm remembering this from a Pyramid or something lol.