The 20 ga. model 12 began in a chambering for 2 1/2 " shells. In 1925 Winchester changed the chambering t0 2 3/4 ". The magazine tube would have to be at least 1 1/4 " longer to hold the 5 shells it normally holds. Be advised that there were 3 different forearm styles (field grade) during the time model 12's were built.
