Richard - depends on how old you are - as Black Hand says, & I'll add, if over 30, sell it and buy a swamped barrel. If in your 20's, have at it with a draw file, nice and long or with BIG rounded teeth like a car body file. With one of those, brand new, the teeth take off turnings, curled pieces of metal, just like a lathe - but slower, of course. I've never filed in a swamped contour, but have made octagonal barrels out of round ones, and round octagonal out of a tapered round - lots of work - too bloody much - buy a swamped barrel.