My name probably popped up because I am sceptical about counterboring. It may well make a rifle shootable that would otherwise be useless. Counterbores I have seen (mostly on Eastern bloc rifles) were of the order of 2 cm, maybe an inch. I think 3 inches is probably not going to work satisfactorily - or if it does, the setup is likely to be
extraordinarily load-sensitive. The reason for this pessimism is that when the bullet is travelling through the counterbored section, it is in free flight. But the barrel is also moving sideways - it jumps and vibrates.
Even if the bullet does not actually touch the sides of the counterbore (in which case it will exit the muzzle at some totally unpredictable angle, and you will be lucky if it hits the target at all) the sideways displacement of the counterbored section relative to the bullet (meaning that the bullet is way off center in the air channel) will have a similar effect to a very uneven crown - there will be a sideways force that will push the bullet way off line.
The best you can do is probably to use a bullet that is as close to the groove diameter as you can get. If the diameter in those last 3 inches is greater than that (i.e. the bullet loses contact with the barrel at some point) then I think the case is hopeless. Note that I said losing contact. Merely having fading rifling is not as bad as that! So (as usual!) clean-shoot, clean-shoot... until you are
really sure that the barrel is
spotless - then make your judgement.
Patrick
Patrick