What about giving it the ability to fire first in battle, defender removes casualties, then normal combat begins. Or what about being able to fire from adjacent territory. Also can it be used as AA and land combat at the same time or do you have to designate in defense.
But doing that does not model the capabilities of these units. The game OOB permits the preemptive firing because it is against just air units that are flying over to attack. It is marginally acceptable to allow this, but realistically these are just basic artillery.
Since they were made as a new unit it stands to bring it fully into the fold as a normal combat unit.
To allow preemptive fire every round is too strong for a 5 IPC unit IMO.
Plus this unit needs to be 3-2-5 as the other units took the other values, so the value of a 5 IPC unit must balance out and preemptive is worth more.