and has relatively low ammo count for a yank plane.
Huh? not even close, with 2000 rounds for the machine guns and an additional 150 rounds for the 20mm cannon, the P-38 ammo count isn't considered low by any stretch of the imagination, even for an American plane. In fact, the P-38's armament is probably one of the most lethal gun packages for an American plane.
ack-ack