I have had six contenders and one G2. Had a problem with one barrel, an old .222 on a new frame. Had to disassemble frame to remove barrel. Probably a tolerance stack up problem. All the others interchange fine. If you look at the underside of the barrels you will notice differences in the little boss on the bottom, some are deeper than others and may need a little dressing down.
Great guns, revolutionized both the investment casting process in gunmaking as well as the long range capabilities of handgunning.