You could make a file called shareammo.mod.gml, and have it execute it on game start:
#define game_start
ultra_set(0, 2, 1)
Then you'd `/loadmod shareammo` once per session and that would be it.
The only friendly fire that there is by default is on explosive weapons, but that is not easily gotten rid of. May want to pick Boiling Veins in such case.