One pair of guns, the inboard set, is staggered aft in the gun bay. Early versions of the P-51 only had four .50-caliber guns (what you'd call today the middle and outer set of guns), but it soon became clear that more firepower was needed.
North American designers could put in another pair adjoining the original four but they ran into a problem with where to put the ammunition. The boxes and feeds for the existing guns had that part of the wing filled to the brim.
The solution was to back up the new set of guns by a few inches — a little less than the length of a .50-caliber bullet — and build in another set of ammo boxes behind.
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