Thank you all for your insights. Myself, the gun dealer and a couple other knowledgeable people I know have been working on this without a definite answer. We have come to the conclusion it is these two boxes of shells. Mind you, we are still guessing that, but.... the problem: fire a 3" shell the gun locks closed.
1.) two new guns , completely different. A single shot 12 gauge and a mossberg pump 12 gauge. Problem occurs in both guns with these 3" shells. 2 3/4 " shells work fine.
2.) Single shot gun, 3" shell fits fine, opens , closes, after firing release jammed ,won't open without forcing release. Spent shell now fits snug in chamber but doesn't seem excessively tight. ( also, put spent shell back in gun, it will open and close.)
3.) Load 3" shell in pump gun, cycles into chamber and ejects fine. Load shell again, cycle into chamber, fires okay, pump action locked, can't eject shell. Only way is to remove barrel, slide it off the shell. Shell still sitting on front of bolt held by ejector arms. Cycle pump and shell ejects fine.
4.) 2 3/4" shells work fine in both guns.
5.) The 3" shells are Winchester super X , 3 " - 1 1/4oz steel BB waterfowl loads. I have two boxes and none work in the guns. ( unfortunately, I haven't been able to find any other 3" shells to try yet.)
6.) Tore both guns down, cleaned , lubed, reassembled, tried again , problem still exists.
7.) Measured shells compared to SAMI spec chart. Can't find any thing obvious. I also compared similar dimensions between the 3" and 2 3/4" shells and can't find anything that appears wrong. The 3" shells do seem to swell to the chamber after firing more noticeable than the 2 3/4" shells, but the Load also seems more.( much louder and much more recoil).
8.) Both guns ARE chambered for 3" shells.
That is all the info I have. You now as much as I do about this. So, thus far, it appears ( to me) the problem is with these shells. I believe the only way to confirm that is to find a different brand of 3" shell and try both guns again. Again, I thank you all for your input. Once I find another shell, I'll let you know what I find.
One other note. It was suggested maybe I don't know how to operate the gun. To that, I can only offer, I am 69 years old, been shooting for 55 of those years, been loading my own ammo longer than I can remember, always repaired my own guns ( I am a master machinist) , so I don't consider myself a "newbie" or lacking knowledge and experience, and I never saw a problem like this.
Thank you all
markf1951