JT.
This kind of devise is not new. The insert would have a short rifled barrel because it wouldn’t touch the barrel in your rifle. I am wondering how they would aim/regulate it. The ones that I have seen were generally for a pistol or rifle cartridge in the same bore as the rifle it is used in. I had a set of the old 410ers that allowed you to shoot 410 shells in your 12 gage
Bill