While I have developed loads that made my double shoot together, I know of no one who will 'alter' a shotgun to make it shoot factory slugs.
; I have little or no faith in factory slugs anyway. I am aware that many hunterw speak highly of Brenneke slugs, but they aren't a big ticket item around here & I have not tested them personally.
; What I do have faith in, is solid round balls. I was able to achieve 8" groups wiht my double gun, at 100 yards, quite easily. Balls may be patched inside a shotshell, like my friends did for their pump duck guns, or using cupped wad bases to hold the ball in the centre of the bore as I did for my double gun.
; Loads must be developed for a given gun to make it shoot the way one wants it to. This takes time, lots of time and money, not only for components but for fuel for the many trips to the range before one finds suitable loads. Ballistics obtainable with smokeless loads will run a 480gr. to 590gr. round ball (depending on size) to well over 1,550fps, which about matches the heavy 7 dram black powder load for 19th century 12's used in Africa. I know for a fact that a 12 bore ball, starting off in excess of 1,400fps at the muzzle, cannot be kept inside a bull moose, from any angle, if shot within 100 yards. Reducing the velocity to 1,100fps was necesary to recover a ball on a 100 yard moose.
: JT - I see you are from Kentucky - this prevents me from offering you physical help regarding working up loads for your gun as I would need it in my possession. As far as I know, no one is currently doing this in the States, but perhaps your post will ellicite a positive responce in that regard. Good luck.