SMG's use low calibre ammunition anyway, so to replicate that without adding some new ammo they use the next best thing.. Pistol ammo.
Also reloading with the ADV.Dupe spawns the anti hack prop which cannot be removed.

SMG's use low calibre ammunition anyway, so to replicate that without adding some new ammo they use the next best thing.. Pistol ammo. Also reloading with the ADV.Dupe spawns the anti hack prop which cannot be removed.