There's always the option to have the barrel lined. The liners aren't that expensive. Don't know what it would cost to have it done though.
How about a 1:20 twist .50 caliber?
http://www.trackofthewolf.com/List/Item.aspx/638/1
This is from a post on another forum.
I epoxy the liners in the drilled out barrels and I have some that have been in for twenty (20) or more years with no sign of loosening.
Recently I have been buying my liners from T. J.'s Gateway Enterprises, 3652 Neltner Road, Alexandria, KY 41001, 859/635-5560. I have been very pleased with them. They are good shooters and he sells them by the inch so you can get whatever length that you need. A caliber and price list is available if you are interested.
And this.
Check with Bobby Hoyt on Fairfield Pa. 717-642-6696. Call after noon east coast time. He has done a number of liners for me and I have always been happy.