I've got 11 H&R rifles and shotguns. All but 4 were put together from stocks, frames and barrels from everywhere. One needed shimming, and one needed the latch shelf touched up. Both 5 minute jobs. The shim job was just a slice of beer can on the pivot. It sounds kinda slovenly, but it works. That's on a .357 tack driver. If the shim gets a little shakey, I just get another brew out of the fridge. AtlLaw here on the forum is an expert on this. If you read the FAQs as previously suggested, there are more permanent remedies to a loose pivot, but none offer more fun.
I don't fool around with spare barrels. I believe every time you take it down and change barrels, you're messing with accuracy. All mine are complete rifles. I would not have a problem changeing barrels on shotguns as long as they were not sighted slug guns.
Grumps, welcome and good luck!

Pete