Originally Posted by
WS6
40% is probably a little premature for this, but 60% and out it is definite.
What happens when a barrel edges into its twilight years---and is still shooting 1-2moa, often maybe even better---is that it has considerable "fire cracking" in the throat area. This resembles a dry lake bed. THe spaces in between often fill with copper and "mask" this, functionally. When you go in there and strip out all of t he copper, the "mortar" between these "bricks" is gone, and 1 of 2 things happen. Closer to 40%, you usually just shoot terrible groups until it fills back in and the gun "settles down", and you motor on, OR, with the mortar gone, large swathes of "bricks" are ripped up and blasted down the bore, and you have, within a few dozens shots, a 1-2moa gun that is now a 3-6moa gun.
You can still clean old barrels, just don't try to take 'em down to bare metal!
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