BigT,
Welcome to ALR. Get a jug of Evapo-Rust from Harbor Freight, Lowe's, etc. Plug the touchhole with a toothpick and pour the barrel full of Evapo-Rust. Leave it sit 5 or 6 hours. Pour the crude out. If it's still rusty, repeat. Evapo-Rust will eventually remove all the rust. If the barrel is browned on the outside, don't let the Evapo-Rust sit on it, it will remove the browning. Once the barrel is clean, look for pits. If it's pitted, your first ball will load and shoot ok, but the second shot may be very hard to push down. Get a new barrel. Life is too short.
-Ron