I've heard there were various thickness barrel sleeves....you might need a thinner one to make it work. Apparently old M12 smiths stocked the various thickness sleeves.
As I recall, there were over a dozen different sleeves available and picking the right one was sometimes a matter of trial and error, until the right fit for that gun was made. A gunsmith familiar with Model 12’s could tell pretty quickly which sleeve was required.
Also wanted to add that headspace is another issue all together.
