I picked up a older 870 20 gauge, clearly marked on the barrel, but when I looked up the numbers on the receiver it came back as a 12 gauge receiver.. did some research and found that in fact Remington did at one time put 20 gauge barrels on 12 gauge receivers.. they flared the receiver end of the barrel to fit.. my question is... Can I just swap out the 20 gauge barrel for a 12 gauge barrel and have a 12 gauge gun???