Comparing my two 1911s in game terms.
They're nearly identical except for damage. The .38 Super gun is slightly lighter (2.6 lb. v 2.8 lb.) and loaded magazines are likewise slightly lighter (0.4 lb. vs 0.5 lb).
Next the .38 Super gets 9+1 shots where .45 ACP gets 7+1.
Lighter, more shots? All win so far.
Next the .38 gets a Rcl number of 2 vs .45's 3. That means more rounds will hit on given roll. Say I need a 15 to hit. Average to hit roll is a 10... that means we make it by 5. That means that with our RoF of 3 that two rounds of .45 will hit but all three of the .38 will.
How about damage?
.38 Super does 3d-1 pi, .45 ACP does 2d pi+.
With ball ammo... 3.5 pts per die average:
.38 3d-1 pi = (3 x 3.5) - 1 = 9.5 points which we round down to 9.
.45 2d pi+ = 2 x 3.5 = 7 points, BUT pi+ gets modified up 50% to 10.5 or 10 points.
So far 45 is slightly ahead per shot, but 38 gives a potential 90 points per magazine against 80. If we hit in the proportions listed in the Rcl example that means that all ten shots will hit with 38 but only five from .45 (90 v 50).
There are lots of stories about .38 Super being developed to penetrate car doors and body armor in the late 1920's. The Elliot Wisbrod Armored Police Vest gets a DR of 6 against pi attacks.
That changes things a bit.
.38 is average of 9, minus 6 so 3 makes it through to the victim.
.45 is average of 7, so only 1 penetrates the armor and the 50% modifier only raises the damage to 1.5 which is rounded back down to 1. Now we're to 30 points against 8 from a full load (50 v 5 from our example).
So, the discerning GURPS pistolero in 1929 will want himself something in .38 Super!