Here is how it works for me (Destiny)

by Earendil, Tuesday, December 23, 2014, 09:25 (3864 days ago) @ nico

This may not be correct, but the theory holds well enough that I haven't noticed it not holding :)

A ammo pickup will be 50% of max capacity rounded down. So,
Max 4 = 2 shells
Max 5 = 2 shells
Max 6 = 3 shells.
Max 7 = 3 shells.
Max 8 = 4 shells.

This is why I carry Truth into PvP combat with armor that gives me 8 shells max, I always come away from ammo drops with 4 shots, and the ammo drop bug has no consequence in PvP.

In PvE I haven't paid really close attention to heavy ammo and whether that principle breaks down. Lately with all the Arc enemies I've been using machine guns instead of Gjallarhorn in PvE. This is where it becomes even more apparent that a single ammo pack gives you 50% max. I think it's the rounding down with really small numbers that gets people.