I know this, but how often do I have to swap? 3 times, 5 times, 1 billion times? I have found nothing that explains it good enough for me.

So I want to learn to speedrun Borderlands 2, since its my favorite game. One issue I have at the moment is, that I don't know how to set up infinite ammo properly. I can do it with single and double barrled Jakobs shotguns, however with a shotgun that has more barrels I don't succed. I watched tutorials on YouTube and also speedruns, but I can't find my mistake. Some help would be really nice. :)