Being honest, although I wouldn't go the route of ammo counting I see no reason you couldn't if that was what you wanted. I agree that the John Woo style outlook to ammo is nicer but it would take only a little thought by the ref to change that.
Assume an SMG fires semi-auto by default (3 round burst) from a 30 round capacity, the weapon will need to be reloaded after 10 attacks. If the firer changes to fully auto then the ammo cost goes up to 10 (or 5 if you prefer) increasing the damage by 1 and reducing the crit by 1 but decreasing the number of attacks before reloading and adding a black dice for recoil (if appropriate). If the firer switches to single shot you get more attacks but reduce the damage by 1 and increase the crit by 1.
For weapons which only fire on one mode the amount of attacks is fixed.
E