Well this is hardly a DayZ issue, it's an engine issue, to change it you'd have to rewrite the whole weapon-magazine-ammo cfg system and add much more code, or find a clever work around ( and that might be the case at some point in development ). To some degree it's already going that way with the convert mags thingy ( workaround ) so i have faith, but to see a proper complex system you'd have to wait for a standalone release ( arma3 doesn't seem like it's going to have a different ammo management for now ).