Quote Originally Posted by DAVALI View Post
I recently brought back to life a BSA light where the muzzle end of the rifling had rusted away due to being stored vertically for some years in a damp shed by drilling the rust out until clean rifling was noted, I checked the rifling by pushing a pellet, after removing the loading tap, through using a length of heavy gauge strimmer line checking the rifling indents on the pellet this got the grouping from in excess of two inches at six yards down to less than half an inch.

Dave (the bodger).
well, you had nothing to lose, and got a great improvement, so I'd say that's a good job. Sometime there's nothing else for it if the rust has really taken hold, rather than getting brutal. Counterboring with a slightly larger drill, and then putting a flat end mill down there to make the crown at the end of the counterbore flatter (rather than the 60 degress at the end of a std twist drill) would be a simple improvement on this, but it sounds like your accuracy isn't too bad already.

Rgds - JB