Well Vasquez shot at the alien head the same number of rounds a regular M39 can hold, which means it is the same caliber
Then maybe the pistol is loaded with +P+ rounds ,that would mean a new barrel a new frame + harder recoil springs (those pistol were not made to shot +P rounds)
Same sh*t for Gorman's pistol
We can also suppose that those pistols shoot regual pessure ammo with explosive projectils ,BUT as we can see none of these projectils have exploded on impact with the alien head . They behaved pretty much like FMJ ( a mix of ricochet & penetrations)
In my opinion these guns (including Hick's shotgun) were pretty much personal pieces, purchased & kept in the sake of gun fetishism (a pretty common thing in the militaries, remember Patton's colt 1873 during WWII). And also as a back-up weapon. They were not really miltiary grade weapon.
