You're right, it looks like he's picking up the ammo on the same frame where he hits 0, didn't notice that. Maybe we just found a new bug, let's find some ways to abuse it :D

Pretty quick: https://www.speedrun.com/cncr/run/z5o70pjy

Skip to 33:25 (or 33:17 on the clock): Watch his ammo count while hes killing the apache and the two dudes, it goes down to 3, he keeps on shooting, loops back to 100 and is at 96 where he then manual reloads. Hes cheating unlimited magazines.