Any service personnel will tell you that the uniform you are issued, makes for VERY crappy armor.
As Mister Fork points out, the idea of a 30 second delay to simulate a change of crew members injured could be interesting. I think 30 seconds is heavy handed though. 15 seconds sounds a little more plausible.

The problem I see with this though is, "Well what about a bomber's tail gunner being swapped with a waist gunner? If manned guns can "change", why can't bombers?", type of argument being made.
I think the idea is more along the lines of the ammo boxes around the gun detonating, which takes out the gun.
