Summary
Current consensus is that the ammo calculations in Fallout 1's executable are bugged, thusly not taking into account the DR and AC modifiers properly. (This has been tested by User: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).
Fixes
Item (PRO) changes
Unfortunately, the development team did not make Fallout as easy to mod as Fallout 2, and thus Celestial's AP ammo mod is the only that exists for Fallout that is specifically designed to fix the issue. Current "fixes" consist of simply changes to the ammo stats via the corresponding Proto files. Whether or not this fixes the issue, or has any effect at all, is disputed. {citation needed}
Code edits
The only way to truly fix the issue is by editing Falloutw.exe directly, either through hex-editing or Assembly (ASM) edits.
Details
- Likely what it will need is to replace the current code with a jump to a new location (and possibly NOP of the rest of the original code) and this new code will run proper checks for all ammo stats including AC-Mod, DR-Mod, DT-Mod.