When I polished out my receiver, I used a strip of rag-cloth attached to a wooden dowel (about 1/4" dia. works well). I wound the cloth around the dowel and coated it with Mother's Mag Polish (available in automotive stores...washing and finishing aisles). I put the dowel in a power drill (I don't have a Dremel), and polished away! Start slow, and gently increase the speed.
Make sure you receiver is stripped of all extra parts (ball detent, sight rail, all internals, etc.). Wash the receiver thoroughly in warm soapy water and towel dry.
If your upper tube is gouged, Mag Polish isn't abrasive enough to really get the bad scratches out, however.
"Good? Bad? I'm the guy with the gun..."
Reaper: Team Syndicate
**I make dead people**