Moar Maffs

We "established" in AWB Math that a 9x19mm handgun did more damage than a .45 ACP because you had more ammo and 11 times 90% is more than 8 times 100%.

This is true, if you don't miss any shots and have the time to deliver them.

The normal rate of fire (RoF) for a semi-automatic handgun in GURPS is 3 per second (or turn).  Yes, you can shoot faster and there are rules for that but they have penalties we don't want to introduce here because I don't wanna (though I might anyway).

A .45 ACP Colt Gov't Model has an Acc (accuracy) of 2 and Rcl (recoil) of 3 with a ST (strength) of 10.

A 9x19mm Glock 17 has an Acc of 2, Rcl of 2 with a ST of 8.

ST won't be a factor so we can skip it from now on.

The average person has a DX (dexterity) of 10 and, to be honest, zero training with a handgun.  Default skill level for the average person with Guns/TL7 or TL8 (Pistol) is a whopping 6.  A person who's attended enough class to gain a point in the skill gets a 10.  A 10 on 3d6 is a 50-50 chance with no penalties.

Always remember that GURPS has harsh penalties built into the combat rules, a person at a range can get up to +10 in bonuses because of the lack of stress from being not in actual combat.

In GURPS 3e a point was 200 hours of study and practice.  4e is less well defined.  It gets murkier when you consider that the skill is used for things besides shooting too.

Traditional pistol range is 7 yards, or a -3 penalty.

The untrained person firing without aiming needs to roll a 3 or less at 7 yards to hit a torso.  Here .45 is better because there's literally no chance of getting a second hit.

The minimally trained person's skill of 10 gets a 7 or less chance to hit with the first round without aiming with either gun.  With .45 a roll of 4 or less lands two rounds.  With 9mm a roll of 5 lands two and a roll of 3 lands three!  Potentially 9mm is better, but those aren't good odds, especially considering that a 7 or less is a mere 16.20% chance; but you are more than twice as likely get two hits with a 9x19 than a .45 (4.62% vs 1.85%).

If you take the second to aim first, the +2 to hit does help a bit.  .45 hits on a 9, 6 and 3 now; 9x19 on a 9, 7, and 5.  9 being ten times as likely to get three hits as .45!  Bearing in mind that a 4.62% chance is not that large...

However, are two shots of .45 as good or better than three of 9x19 since the odds are similar.

On average, the .45 will do 20 points of damage and the 9mm 27.

Again, this is against the Torso.

If we were trained to aim-small, miss-small and we aimed as small as possible on the torso, we're really aiming at the vitals.  That's an additional -3 penalty.

Now .45 hits on a 6, 3 and can't.  9mm hits on a 6, 4 and can't.

Average damage for two hits is now 42 vs 54...  This is because there's no difference between pi+ and pi when you hit the vitals, skull or eyes.  Odds are you're wasting two shots from either gun at full RoF at this range/skill level though.

A range of 7 yards might be a tad unrealistic, an assailant is probably charging you.  Probably with a move of 6 yards per second if you include sprint bonuses.

That means all the fun happens at one yard!

Another "odds are" statement.  With a charging assailant, the untrained person is probably going to open up on the bad guy at 7 with all three rounds, then fire again at 1 yard with all three rounds.

Chances to hit are 3, can't and can't at 7 for either gun and 6, 3 and can't for .45 or 6, 4, and can't for 9mm.

The trained person aims at 7 and only gets to fire once the range has closed to 1.  -3 for aiming at the vitals, 0 penalty for range and +2 for Acc.  Chances to hit are 9, 6 and 3 for .45 and 9, 7 and 5 for 9mm.  Just going for the torso changes things to 12, 9 and 6 for the .45 and 12, 10 and 8 for 9mm.

So, yes, in GURPS terms you want to use a 9mm and not a .45.

But wait!  That's all ball ammo!  What about hollow-points?  Same progression with narrower margins on the torso, same margins on vitals.