Win Mod 12s in 16 ga w/ short chambers are notoriously finicky about which 2-3/4" shells they will cycle, but the guns are plenty strong enough to handle the shells. As noted the problem is the ejection port, rather than the chamber length. Try a few brands of shells in the M12 and see if any of them will cycle successfully. If you find one that works, just use that and leave the gun alone.