I supposed ack was 12.7mm weapons like those of the P51. Flying P51 I need to concentrate the fire one second or more on the enemy to do any substantial damage, but actually, 3 or 4 pings of the ack fire can make you a wingless pilot. And as you move fast near the enemy base, the ack can't concentrate the fire in a single point of your plane like a P51 can do. So, IMO, ack has actually much bigger caliber than P51 machineguns, or its initial proyectile speed is much higher.
If you try to attack those acks with a fighter using your guns will result with you killed 90% of times, and using rockets will result with 99% of missed targets (you'll need a direct hit). Those acks are not armoured Tigers, so, a near hit of a rocket should put the ack crew out of action, at least for several seconds or minutes.