I'm not so sure a change in heavy ammo would necessarily harm the other guns it's popular on besides the gat/flak. At least after some consideration of adding say a 20% damage decrease instead of (or in addition to) the 25% reduced muzzle velocity.
The manticore and carronades both benefit far beyond the gatling and flak from the 80% recoil reduction, which for those weapons makes up for that damage loss. The point of this change being to extend the engage time for the gatling to peel armor at max distance because of the damage reduction. It would also make it take an extra reload of flak for most ships to down by using it. This turns these medium-long range engagements to a battle of positioning, precision, and time on attack. Something I feel there isn't a lot of outside ranked matches.
Besides increasing engage time and offering risk/reward for firing non heavy flak, this also promotes the use of gunners. Gunners who can bring damage enhancing ammunition for these weapons to switch to once their opponent is close enough to engage with full effect. Also, I could see higher use of buff engineers to offset the damage reduction rather than make it all the more deadly (which I admit, I'm a huge fan of doing... 30-50 round armor peel is devastating at max range with a good flak gunner waiting).
Anyhow, just thought I'd share the idea.
While merc does give another option, people seldom use it for, I guess the arc?
That arc is the main reason I see it not used much. However it also takes longer to achieve a strip compared to a good shot on the gat due to the need to reload. Also a single miss is devastating to it's efficiency.