Rudy.... have you ever worked with a chamber ring delayed blow back system?,,,  I am thinking of machining a shallow tapered groove just ahead of the case web area in my 9mm carbine... for the case to momentarily expand into, then be resized back at it moves rearward, slowing down the bolt... as always, I appreciate your comments... thank you

No sir, I haven't....my initial thought is that it would probably just rip the case in half.......It would have to be perfectly located to hit the brass at just the right point to not allow a blowout from pressure....or worse cause the case to hang and not cycle......
But just because it sounds like it is insane doesn't make it so.,.......have you found any published data/info on this beastie?

I see where it was employed in seecamp pistols.... but only with .25 auto, 32 auto, and 380. the description I found mentioned a restriction at the chamber mouth, not a groove....the best I can equate it to would be a resizing die at the chamber mouth...........it would eliminate the ability to cut the chamber with a conventional reamer......chamber would have to be cut with a boring bare...and potentially held to incredibly tight tolerances............. might be fun to try though....if I ever get any spare time.....

thank you for the reply... I had a Seecamp 32 which had the chamber depression... it hand ground and was pretty crude...my Cousin is a good amateur machinist and will cut the boring bar ( possibly use a tool post grinder with a diamond cherry) if I decide to play with it...not sure how case hardened a nitrided chamber is ... if it works, sure is cheaper than a short piston system... please keep us informed if you decide to do one... regards

I was reading about the Scheel roller-delayed blowback buffer system and was reminded about the chamber ring delayed-blowback.  This might be more ideal in rimfire applications where the brass is generally not reused.  Just curious if you or anyone else has attempted this.  I think there are many variations to the profile of the chamber ring that maybe the computer modeling available today could aid in developing.  Otherwise one would have to have a stack of 9mm barrels to "try it out".

Check out the Mann pistol in 25ACP. It intentionally bulged the cases in the middle of the body as you guys are talking about, but with 25ACP.

This little trick was done to help reduce the weight of the bolt, and keep the pistol very small and light.

Incredibly brilliant considering this pistol was made in the early 1920's