Ahh I've got it. There is a case with calling weapons with keys, if you have a responding weapon in your prepared slot, the game initiates a swap instead of pulling from inventory, just because it is faster. We usually miss this because the prepared slot is the best place to store an exotic weapon of any kind, or a chainsaw, which is called by pressing zero key, and we usually don't have a loaded BFG on swap to hit the point with two BFGs, while it's a pretty common case with shotguns, since if you take Juggler early, you can storm your enemies with shellfire from more than a single loaded shotgun in inventory, which is highly useful early on. To make it work, you have to have two weapons to place in your hands that are not shotguns, because of the said effect that a swap is checked before inventory.