The local gun store has a wide range of adapters, but the actual barrel length in front of the ctg. is only an inch to 2 1/2" depending on the actual ctg. chambered in them. These "things" have been out for a number of years, even back to the 80's IIRC. That one was normally for .30/06 and 300mag. for use with a factory .32 S&W round, so the entire barrel was used.
The ones on the shelf at the store are usually for 3"chambers and run from .22LR & .22 mag up to 20bore in the ones fitting 12 guage shotguns.
Word of caution though. The large fat firing pin of a 12 bore might have trouble with the primers of higher pressure ctgs like the .357 and .44 mag. Ctgs.
I might pick one up in 9mm or.45 Auto just because the rifled section will be longer, but then my rifled 12 bore stays at home or goes camping with me. I don't pack it hunting, but could for deer. Hmmmmm.