This really isn't just a collision issue, it happens anytime the guns are damaged by whatever means.
Got shot in a Hurri IIC 4X20MM and had two left and two right MG's show up as "damaged."
Not sure if there's an additional problem but I also had a right 20mm in the damage report but with no corresponding visual damage on the wing. I'm not positive but am pretty sure that you always have visual indications the gun is damaged. I also had a left 20mm damaged but that did show a corresponding hole in the left wing near the gun.