So... I stumbled upon a thing. Turns out that if you're reloading when you enter the water your gun is reloaded instantly. Turns out once that happens any interruption of your reloading animation also causes your weapon to refill instantly - like melee attacks. This seems to go away if the game loads again.
This is really useful for a player who likes to use weapons up close and personal. Good find!
As far as I can tell I can't see any real use for this in a speedrun outside of going to Rabotnikov from Golyak, but only in NG if the plan is to use the Hothead on him (and limpets followed up by the shotgun may be better anyway).
That said, it does help with grinding up Firearms EXP (for a NG+ character or going for the 4* Agent achievements) by respawning gangs over and over and heading straight to Garcia with an HMG and the shotgun. The HMG really benefits from the glitch and makes it considerably less painful overall. A 100% route would probably have some use for this as well at some point while collecting 800 orbs.