Caleb, I have been researching the two models for about 9 years and as best as I can determine, all receivers are the same size. There are internal differences and different markings on the outside, but a .410 M220 has the same size receiver as a 12 gauge. Barrels will interchange, but......early barrels will fit on all receivers, but will not cock the hammer (on late receivers) when opened if they do not have the "late" cocking stud on the right rear of the barrel lug. Late barrels with this lug, will function on early receivers, because the top lever cocks the "striker" when the action is opened. Now again, generally they will fit or a gunsmith can fit them. I took a hornet barrel off a M94 Hammer gun, and it fit perfect on a M220 20 gauge action I had. That was good luck on my part. I recently purchased a .30-30 barrel that has been converted to .219 Zipper. You can tell from the fresh machining marks that it was fitted to an action at some point in time. My first gun I put it on, it was so loose, it would not lock in place. The 2nd gun i put it on, it locked in place, but had a gap and was loose. The 3rd action I tried it on, it fit perfect and locked in place tight. However, the forearm that came with that rifle, will not fit the .219 barrel. I have an extra forearm that I will ask my gunsmith to fit to the zipper barrel as I do not want to modified the existing forearm, as then most likely, it would not continue to fit the existing barrel. As to the forearms being gauge/caliber specific, I can say that all rifle forearms I have examined are the same size in the barrel channel. The rifle forearm channel is good for the .410. The other forearms have channels to fit the different barrels. While a 12 gauge forearm may lock up on a 20 gauge gun, you will have a large gap between the wood and the barrel. The are many different styles of forearms for rifle and shotguns. I hope this helps you some. I go into detail on this in my book. Best wishes, Garnett P.S. There have been a couple of 28 gauges guns on Gunbroker recently. One has a cut barrel. The 28 gauges had 28 inch barrels. Length of pull for the stocks on all models examined is 14 inches. Lots of stocks are shortened.
