Current consensus is that the ammo calculations in the executable for Fallout 1 are bugged, thusly not taking into account the DR and AC modifiers properly. (This has been tested by Sduibek and Wasteland Ghost, likely by many others as well.) Fallout 2 has similar issues although is not as broken as Fallout in this regard. Various ammo mods exist for Fallout 2 to address this issue such as Cubik's Ammo Mod and YAAM (Yet Another Ammo Mod).