Over 50 years ago, I forgot to clean the bore of my .22-250 Rem 700 after varmint season and had propped it next to metal sanitary stack in the walk-in closet. When I decided to check it in the spring, I could barely see through barrel, due to corrosion. I was sick about it.
A rod wouldn't go through it, so had to plug the bore and put solvent and oil in it for a few days. When I finally got a rod through it and looked at the bore, it was severely pitted.
I quickly put a few handloads together and drove to the range, where I put up a target at 200 yards, then went to the firing line and laid prone without a rest. I expected the bullets to tumble and probably miss the target.
To my amazement, I fired a 5-shot group that measured 5/8". The worst thing was that I couldn't remember the load I'd put together a few days before and didn't even have any left.
I shot that rifle for a few months, but sold it to a guy who didn't care about the bore, then got a new one.