The only upgrades seen in Fallout 3 are scopes, the occasional use of silencers, and the use of extended magazines.
Colt Rangemaster hunting rifle → Scoped hunting rifle.AK-112 assault rifle → Assault rifle (exp.In Fallout 2, you can pay a mechanic to upgrade certain weapons for you. Only Smitty can perform modifications in Fallout, and the only weapon he can upgrade is the Winchester P94 into a turbo plasma rifle.