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%)
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.