I have found only a hand full of barrels will require fitting.
There was one sport barrel I had, brand New, and it would not fit any of the model 41 frames I had, and we are talking a bunch.
Never force the trigger guard closed if you do find one that does not quite lock up. It's not too hard to remove the material from the right spot on the locking lug to get it to seat. Best let a gunsmith, or you would need to send the whole mess frame and barrel to S&W. They charged me $75.00 quite a few years back.