I have never powdercoated a gun barrel, but i did have a paintball gun barrel powdercoated by a local company. It was aluminum. I would probably bead blast the barrel first, to give the powder a little something to stick to.
The 416R stainless that most manufacturers use will rust. Especially the bead blast of the T/C stainless guns that I have. The new this year "weathershield' coating that T/C is using on the "Triumpth" rifle will protect it from about anything. The weathershield coating is applied by Doug Burche who also supplies the Bore coat kits and his application is a "Stainless Cerokote".
I am personnally very impressed with this new coating. T/C will likely offer this coating in a blue finish in 2008.
The cost of stainless steel has risen sharply this year and the weathershield coating will enable T/C to manufacture a rifle in chrome-moly and have the weather shield coating applied with about the same cost as stainless and offer the gun buyer a weather resistant rifle.