50 cal mount locations

[caldera]

I am wondering why on certain U.S. planes, mgs are mounted flush with the wing edge and others protrude a bit?

P-51 and F4U 50 cals mount flush
F-6F, P-47 and P-38 50s stick out.

Is it space requirements, manufacturer preference or something else? Maybe a slight aerodynamic advantage to flush mounting?

[Blooz]

Depends on the ammo storage and feeding arrangement.

[caldera]

Thanks Blooz.