I have a M12 16 guage about 1923 vintage that will eject fully loaded shells everytime, but fired rounds will extract but not eject from the chamber. The shotgun has been totally stripped, cleaned and reassembled. There are modest signs of wear, but not heavily worn. I suspect a worn ejector, but it makes little sense to me that unfired shells will eject perfectly. Any ideas?

Take it to a qualified gunsmith and ask him to measure the length of the chamber.

Yes, you could have an obsolete length chamber. Firing too long of shell in a shorter chamber can cause all kinds of problems, least of which can be failure to eject. What length shell are you shooting in it?

Most 16 gauges from the pre-War era were chambered for 65mm (2 9/16") shells,. 2 3/4" shells worked fine in them, as long as they were roll crimped with a top wad. The change over to star crimped shells changed all that, for not only were the fired cases too long for the chamber, causing potential pressure problems, but the fired cases were at the same time too long to clear the ejection port.
I have a Browning A5 with exactly the same problem.

If you are more than a little interested in shooting this relic, you should either get some 65mm shells (which are difficult, but not impossible to find) or have the chamber and ejection port lengthened to accomodate the modern shotshell.

Actually, the shells are quite easy to get, just not at your local store. Go to www.16ga.com for a whole list of suppliers. With doubles, it is easy to run in a chamber reamer, but the old repeaters often need to have the ejection port milled/ground. I would opt for ordering shells, which isn't expensive compared to retail outlets for 16 gauge.

I believe it is a relatively easy mod to get the M12 to function with 2-3/4 shells, Winchester and Browning used to do it for a nominal charge. Probably hard to find a smith today old enough to remember how to do it. You need an old gunsmithing book?
C.G.B.

I had a 16 GA model 12 for 30+ years that was a bit older than yours. It would not eject Winchester or Federal shells (they would catch in the ejection port) but it did fine with black Remingtons (game loads, I believe) - you might try a few different brands of shells before you go to the expense of altering the gun.

This was a common alteration which we made in our shop. Win. 12, 16 ga. and 20 ga. were changed after the plastic cases came out. Brn. 16 ga. short chambers take much more conversion to operate. I don't know who has written up the conversions for the trade. Find an older gunsmith and he will probably know the steps to change the shotguns. I will show the type of tool I made in 1965 to check the chambers and chokes. It is fast to use at a gun show to know if a shotgun is what you are looking to buy. By bending the lower arm on the tool it would measure 410's to 10 ga. chambers and chokes. Place the tool in the barrel, adjust the dial indicator to zero and slowly draw the tool out. You can see the forcing cone, check for a swelled chamber, and the length all in one operation. At one time I had the factory measurements written down, but lost them along the way. You may think that this tool isn't accurate. We had the 3 ft long micrometer to make the full barrel check, but my simple tool was used most of the time and by checking the prongs with the micrometer it would be within .001-.002 of the true measurement. I used a Starrett dial indicator for my tool.

If you want a collectors gun don't alter, but if you want to hunt with these old shotguns get them altered for the newer shells.

I'm not old, yet, but I have modified many of these chambers in the past. You can use a long forcing cone reamer to accomplish this. You can lengthen the chamber and forcing cone in one cut. FWIW, lengthening the forcing cone by itself probably produces the biggest improvement in performance of anything done to a shotgun.

+1 on lengthening the forcing cone,the groups will turn out better and preconceived recoil will be less.