15mm is a bit over half an inch. The 50cal round can punch through 2 separate plates of 1/4 inch steel armor plating placed 1 meter from each other, with the shot fired from 1000 yards away. I've seen this done with a sniper rifle on a show on TV [discovery channel demonstrated it], but with the same basic round as is in the Browning guns.
I think it's just the thin skin of the turret being shot up, but who knows, I could be wrong.