Slugging the bore to find the tight spot
It's known among .22 bench rest shooters that for a barrel to exhibit the best possible accuracy, the tightest spot in the bore needs to be at the muzzle. As I understand it, their procedure is to slug the bore, find a tight spot, and cut it there. Alternatively, they will lap the bore except the final bit near the muzzle. This subtly increases the diameter everywhere except the muzzle.
I recently bought a 40X, and the barrel was very long. I wanted to shorten it but was worried about cutting in the wrong place. Insomnia struck again the night before last, so I was in the shop at 3 AM dicking around with it. I figured out how to use a .22 pellet as the slug. I drilled an 0.062" hole through the center and tapped a pointed brass jag into the hole from the back side. This allowed me to upset the slug just by tapping on the jag with a hammer. It was simple to get the slug resized up 0.002" - 0.003" every pass.
I probably ran 50 passes trying to figure out what I was doing. By the end, I landed on the process of two dry patches followed by a freshly reset slug. I ran the slugs from both ends. It might be more useful to slug from the muzzle, but I like doing from both ends to confirm that what I'm seeing is real.
Much of what I felt was subtle and subjective. Going from the chamber, I think I felt it loosening in the center and tightening up for the last 6". What was certain is that the last 2" at the muzzle is the tightest spot on the entire bore. Checking from the muzzle didn't indicate the gradual tightening of the last 6", but it was even more obvious that the muzzle is the tight spot. That even makes sense. If the muzzle sizes the slug to its minimum dimension immediately it will be very difficult to detect the gradual tightening of the bore over the last 6" because it's already undersize.
I decided not to shorten the barrel at all. I could maybe have gotten away with 4-6" since I felt it getting tighter there, but I decided to just recrown it. My guess is that someone lapped the bore to get some "choke", and it seems stupid to undo their work. The gun is shooting so well already (1/2" at 100 yards), I'd likely have just made it worse.
Here it is in the AICS is now shares with my .260.
http://i27.photobucket.com/albums/c1...psykf3xhde.jpg
