The recoil lug has a protrusion stamped into it to align with a notch in the front of the receiver, and the fit is pretty close so it's no problem unless the lug has been bedded. If so, I would probably want to pin the lug as well to make sure it would fit. As for swapping barrels back and forth, it gets kind of tricky to make sure your dies are adjusted to your chamber. I usually will set the headspace on a couple of cases that were just sized with the die that's going to be used, so the bolt just drops with a slight feel on the case. That will get it real close, but it's still not optimum. It's best to not do that very often.