Ok, the discussion got me interested in proving it, I've got a couple films that do just that.
I spent some time in the training arena testing it several times. My testbed was a P-47-D30 with 8 guns and 425 rounds per gun. This plane was shot on the runway (by an LVT) until a gun was reported damaged. The guns were always fired using the trigger that fires both primary and secondary groups simultaneously. Here's what I noticed.
1. This problem doesn't always occur. It may be specific to certain guns. On one flight I had one gun damaged and all the remaining guns ran out of ammunition at the same time.
2. When the problem does occur, it seems as if one gun will have a significant amount of ammunition remaining after the others run dry. This is in excess of 100 rounds as indicated by the round counters. Additionally it appears as though all of the guns in a group of 4 (P47's have 2 groups of 4 guns) may be disrupted to some extent, as the undamaged group ran out of ammo entirely before 3 guns in the damaged group stopped firing.
3. The one gun with remaining ammunition (due to reduced rate of fire?) is on the opposite wing from the damaged gun. This is clear from the tracers and hit sprites coming from that side of the plane. My assumption is that it is paired with the damaged gun.
4. This is not just a graphics or sound glitch. The last gun is definitely firing and dealing damage. I shot down a squadmate with it to prove it.
Here is a link to a film of this test if anyone is interested in viewing.
http://pages.sbcglobal.net/rollio/_images/DamagedGun.ahf