What the devil is this thing?

[crkckr]

I've had this thing in a box of stuff I picked up in trade 40+ years ago (all the stuff I got was pretty old then, so no telling it's age). I have no idea what is! There are no markings on it at all but it's the same base size as a .308 cartridge... and no, I've never put in any of my rifles! A .22 LR cartridge will fit in the base but there's no rifling in the end, plus I've seen sub caliber inserts before and they were off set so a firing pin would hit the rim... in this case it would just leave a dent in the center, accomplishing nothing. The center hole size is probably a coincidence. So, any guesses what it is?
Cheers,
crkckr

 

[sea2summit]

COAL gauge of some kind?

 

[Code4]

Sub calibre chamber. You put it in your rifle with a loaded .22lr/Hornet round [whatever is was designed to hold) and use it to dispatch wounded game. Common in europe. There are versions for shotguns as well with 410 shells etc etc.. There were also special sub-sonic rounds for it. I forget their name.
Possibly manufactured by Lothar Walther.

 

[AZShooter]

Looks like a chamber insert to me too.

I remember there was a Smith and Wesson model 53 with what they called Owens inserts. It was the 22 Jet/22 LR revolver. Read it in my father's old gun digest in the early 60s. See part here:
https://www.gunpartscorp.com/products/880940

One of my father's friends made his own insert for 308 to fit in his 30-06.