Please give me some help. I want to buy a Winchester Model 12 in 12 guage. Assuming the barrel is in good shape, I will eventually want to buy a second barrel to shorten to 18-20" for whatever reason. On Model 12's, is the barrel a simple drop-in switch, or are there certain things to watch for or that have to be fitted? I want one, but just don't know that much about them. Thanks.