In megamek, there used to be an option called 'Limit ammo types by year'. I can't quite remember which tab it is in, and I don't know if it still exists.
That option will use the introduction year of the each unit to determine what ammo it can carry, even if the presence of another unit clearly defines the battle as being set later. If you leave it on, no Narc ammo on your 3025 vintage missile boats, even if you have a 3050 Raven with a beacon launcher in the force, for example.
