The variable would be something like gun_muzzle_firing_#, where # is the index of the muzzlePosition. The value will be 1 when this muzzle is firing, and 0 otherwise. This will allow multi-barreled guns to show individual barrels recoiling, and trigger muzzle flash visibility at the correct muzzle.

This will require some tweaking of the vehicle animation system due to the # meaning something other than a part number, and this will only be useable within the gun JSON.

I ended up using gun_muzzle_#_firing to avoid changing the system too much.