There are a couple of ways to upgrade a weapon, but its obviously more limited than what we see in video games.

Most of what you can do to affect the damage is based on altering the bullets, not the guns. In the US at least, there is a long history of "wildcat cartridges" with amateurs creating new calibers of ammunition and loading standard rounds differently. Ke=1/2mV^2 (Kinetic Energy equals Half the mass time Velocity-Squared) So most plus power rounds accept a slightly smaller and lighter bullet, but with more powder charge behind it to get a higher velocity. Of course, lighter bullets don't retain inertia as well, so accuracy and range may suffer - as does the shooter who will experience greater recoil force. One could also turn to diffrent bullet types and shapes - Hollow Point, Steel Core, and so on - for additional affect.

As to weapons themselves - a longer barrel means more time for the gas to expand and push the bullet. Stronger construction/ and recoil springs will allow it to survive firing more powerful bullets. (Make the springs too strong, and it won't relibly cycle with weak loads. Revolvers have an advantage of tolerating any load without worying if its capable of cycling the action - that is done by the user)

High capacity magazines are possible of course, but larger mags means longer springs that wear out and thus less reliablity. It was pretty common, in vietnam at least, to only load 30 round M-16 mags with 28 cartridges. Changing the capacity of a revolver, is probably impossible, as is changing its caliber - but most are tolerlant of firing ammunition of varying quality and stregnth without problem.

Upgrading weapons makes a bit more sense in context of a Fallout type world. In this scenario the old guns are rusty/bent/leaky and thus show sub-optimal performance, and mixing parts is restoring them to ideal condition.

Admitedly the Siaga-12 is an AKM modified to fire 12 guage shotshells, and there are a few multi-caliber guns out there. with conversion kits. The famous Desert Eagle cn be switched between .357, .44, and .50. The Medusa-47 revolver is a unique design that can accept around 25 diffrent types of ammunition in the 9mm/.357 range without modification

There is a fine line between hobby and obsession. I seem to have lost sight of it some time ago.