http://www.foxridgeoutfitters.com/section.cfm?section=16 Fox Ridge Outfitters sells the kind of barrels you're looking for. Also
Green Mountain Barrel Company sells replacement barrels that will fit the Hawkins and Renegade in both .50 & .54 calibers. You can even get them in .58 caliber and have a real big bore rifle. The Green Mountain Barrels are excellent barrels. They come in 1-28, 1-48 (I think) and 1:70 twist depending on whether you want to shoot sabots, conicals, or roundball. I would strongly recommend the GMB. Give them a call, although they are back ordered on certain model.