Take an empty double barrel shotgun into your hands, put your shotty ammo pile on the hotbar, press it to reload (the animation should play exactly as intended the first time) now spam the hotbar that has the ammo pile in it, (I imagine you could probably get a program to spam that number when you hit a certain hotkey but I would have to test it out to see if it works) as you fire you will notice that the ammo will keep being loaded into it without the animation playing, I've attached a screenshot of what I did with 15 rounds, I only once saw the animation play at the beginning, after that it was like I was using a semi auto shotgun with a 15 round mag. I also tested this exact method with a mosin, but because it only has one round in the chamber at a time it made a clicking noise the hole time, and after each shot I had to watch the bolt action animation, but it still worked without having to watch a reload animation every round.

I tried this with a glock, and near as I can tell it does not work, I think the mosin and double barrel (possibly the izh12 and blaze as they run with the same theme of weapon) may be the only ones it works with, I will need to test it more, but I would highly recommend you removing these guns until the problem is solved, the double barrel specifically would be devastating if it remains in the game.

PS: I'm making this private because its very easy to reproduce and very dangerous, also near as I can tell no one knows about it yet, and I would like to keep it that way.