My father, who is a machinist, built his own shotgun barrel to convert his ML rifle (a TC Greyhawk) into a 12 gauge.

I have a couple (12 and 20 gauge) on order with him...stocks will be made from American Cherry.

His favorite turkey load (check your own manufacturers recommendations) is 120 grain Pyrodex, an overpowder card, 2 fiber wads, 2 oz of #4 and #6 mixed (about 50-50) in a "nylon" pouch, with an overshot card, ignited by a Federal M209 primer.

With his full choke, he gets kill patterns out to 40 yards (that load comes close to killing on the backend as well).

The "nylon" pouch is made from pantyhose sewed together with black nylon thread. Use a meat scale ($20 at Walmart) to measure the shot.