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.