I have a set that I am going to fit to an old Belgian backaction hammer SXS 12 Ga. These are in .45-70.
They are 18" long. There is no provision for extraction, so this will need to be milled in and fabricated.
I tried to get Ace to make longer ones but failed.
I have been toying with ideas for regulating using long bushings at the muzzle end to keep a profile of a minimum 22 inch barrel to have correct balance and reasonable sight radius.
Quality wise, the two inserts look good, but, are inconsistent in diameter at the forward fitting swell and at the outside dims that fit the 12 ga chamber. In honesty, I do admit that the inserts were ordered mayber two years apart. I had a single shot in mind earlier, but was inspired by this Site to try our making the double!!! One of the inserts will fit with pressure in typical European / English bores, the other is just a hair larger and will not. They both fit in standard US diameters for internal bore deminsions. I have a Stoeger Couch SXS that they fit. I will try them out first there (not perminent) for shooting test before I mess over the old hammer gun.