If those small AP bullets do what anti tank penetrators do, then if striking at an angle, they either deflect and deform (denting the surface possibly), or they penetrate, with the projectile bending/turning into the surface. Even if it bends inwards, I'm sure the hole is not completely round, nor oval. Probably slightly heart-shaped (but rounded on the upper edge). Either way, it is insignificant. Round is pretty close for most situations. The rivet issue is more important. Even if ENEMY planes may be far away, friendly planes may fly very close, close enough to see. More so, seeing one's own plane from the cockpit (especially noticeable in twin engined or more). That's close enough to notice.
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