I own quite a few Model 12's and have for a long time. Without actually counting, there's gotta be at least 20 or more in my safe as well a couple of 42's and a few 1897's, 1901 and 1903. I've hunted with a Model 12 since I was a boy. No other shotgun comes up on target like a model 12. The only other shotgun that I've found that I can pick up and shoot like a Model 12 is my old Browning Superposed.
My oldest M12 is two very early 2nd year 20 gauges. Both these guns are nearing 100 years old and are still shooting. Also a first year 12 ga, 1914, and it's still going strong. I have a couple of 16 ga, one is very early with 2 9/16 chambers and the other 1948. The very early 20 ga has a 25" barrel and the early 16 ga has a 26" barrel. These little gems are more than a pleasure to shoot. Hell, I shoot Walmart specials in these guns like you never saw before.
Like someone already said, the 16 ga, 20 ga and the 28 ga are all built on the same frame. Unlike the 870, it's 16 ga is built on a 12 ga frame.
I even take my trap guns out bird hunting. They work fine. My favorite trap gun is one from 1937, solid rib barrel and my newest trap gun is 1963.
Anyways, the business about the M12 wearing out and the bolt flying open is not true. Pretty much can't wear out something that can be easily repaired or rebuilt. The nub on the end of the action slide presses the bolt up and into the locking notch that's in the top of the receiver, When that nub is not clean and lubricated, over the years the top of the nub will wear a tad causing the bolt to droop in the lock-up positon. To check for this, press up through the shell lifter with your middle finger to see if the bolt is locked up tight or if there's upward movement. When there's excessive upward movement the rear of the bolt is not fully contacting the surface of the lock-up notch in the top of the receiver. When the droop gets worse and a few thousand or so rounds, the lock-up notch gets rounded off.
To repair this is not difficult. Have a welder, with a mig, build up some weld on the top of the nub of the action slide. Blacksmith the lock-up notch back in shape by peening the metal. Reassemble and your good to go for another 20 or so years or more.
Another check point on a Model 12 is the takedown adjustment sleeve on the chamber end of the barrel. Take the gun down and you'll see the adjustment sleeve on the receiver extension. It's preferrable if the sleeve is still on the first or second notch. First notch, all the adjustment teeth will be to the left of the locking dog, looking at it with muzzle down.
That's it, I'm tired and going to bed. If you need more info on the Model 12, just ask and I'll help where I can/
Regards:
Rod