I have several "long range" slug guns as follows:
​- Pedersoli Gibbs which shoots well enough right out of the box to be able to win a world championships with. I am not sure who makes their barrels although several folks told me they have a custom shot in house to do them. They have very shallow rifling.
​- 50 caliber Douglass XX stainless which can shoot 5 mini balls into one hole at 300 yards. I use a 540 grain Creedmoor nose bullet, fiber wad and 90 grains of FFG. I put a Unertl 10 powder scope on it. Barrel is 1 1/2 across flats, 38 inches long.
-  45 caliber green Mtn 1 in 18 built in an under hammer, 42 inch long with approximate same load as above. Not quite as accurate as it has a tang sight.
​- 45 caliber gain twist 1 in 18 -> 1 in 15 barrel with NO marking, has false muzzle, bullet starter and hooked patent breech, 1 1/8 across flats, 36 inches long. It has a 15 power Unertl scope and shoots about the same as the other one at 300 yards.

​I have tried paper patching and I am lucky to hit the side of the mtn.!!!!!!!! I use Doug Knoel's lube which is pretty darn great. Wipe between each shot 1 wet, 1 dry.

Hope this helps some. About the only one to buy is the Pedersoli Gibbs or from a custom maker. John

There are a few around far and away you might look up Lee Shaver as he is involved with the sport and makes a rifle for this game.
Good Luck!

Richard