Here is an initial stab at re-implementing the Ammo rule from SAE.

Ammunition is tracked for each battery, not per weapon mount. Unless otherwise noted, a weapon battery has unlimited ammunition. Batteries with limited ammunition have the amount listed in the battery display, just below the arc designations. The number given indicates the maximum number of times weapons in the battery may be fired (e.g. “Ammo: 10” indicates that the weapons in the battery may be fired up to a total of 10 times). Regardless of how much ammo is available, each weapon in the battery may only be fired once per game turn.

Each time a weapon in the battery is fired, reduce the ammunition value by 1. Once the ammo value has reached zero, the battery is out of ammunition, and its weapons may not be used any further. (Note that one point of ammo is spent for each weapon fired, not per to-hit die rolled; e.g. when a weapon with a ROF value of 3 is fired, one point of ammo is used, not three.)

Ammo = 1x Weapons: Multiplier x0.2 (same as Expendable)
Ammo = 2x Weapons: Multiplier x0.3
Ammo = 3x Weapons: Multiplier x0.4
etc.

Batteries equipped with Ammo use the same hull size modifier as Expendable weapons.