That´s kind an odd rule.
Why would I need to fire twice as many SRM-6 shots a month to stay current on a BLR-1G BattleMaster than on a CRD-3R Crusader, even though the latter has twice as many SRM-6 launchers as the former? And why would it take that many more machine gun bursts to maintain gunnery skills on either ´Mech than SRM-6 shots?
I would think a more plausible rule would be "ammunition expenditure for live fire exercises needed to maintain gunnery skill for a month equals expenditure of all the unit´s weapons at maximum rate of fire for X turns", X being a reasonable number somewhere between 2 and 10.
E.g. for X=5, you´d need 10 shots of Ultra AC ammo, 30 shots of RAC ammo and 5 shots of any other kind of ammo (except AMS) for all the unit´s weapons. And maybe for the sake of simplicity, the rate of fire for Streak SRM is assumed to be .5 or .75 or something like that.
On second thought, perhaps the X could also be different for different skill ranks? Surely it takes more practice to stay on Elite than to stay on Green?