The AC-5

[Sanjuro]

AFAIK the C5As are being retired because the airframes are approaching the end of their fatigue life- giving them a new role which had a higher level of airframe abuse would be problematic.
As one of the calmer posts on spacebattles said, the advantage of a bigger airframe for gunship use would be in longer loiter time, carrying more ammunition and more fuel. Weapons on both sides would mean that 50% of the weapons would be completely wasted in each engagement, not to mention the extra crew training time required.