Since fuel can bombed and reduced to 75% available.  why not make it a simular way for ammo/bombs.
100% up all ammo/bombs available
75% up  Bombs 500kg/1000bls or less available,  75% cannon ammo load
50% up  "    "   250kg/500lbs   or less available,  50% cannon ammo load
25%  up  "   "   100kg/250lbs   or less available,  25% cannon ammo load
as it is now ammo had ZERO effect on fighters which it could in RL 
have a affect on ammo quality and quantity, especially the high end
cannons.
This would also leave field defenders with atleast 250 lb bombs to defend with.