Was shooting my S&W 317 .22 one day, and after 30 or 40 rds. my shots were keyholing. When I got home, a close examination showed that the Remington bulk pack Cyclone (lead) had severely leaded up the bore. Bullets could no longer bite into the rifling.
Cleaned it out with some steel scouring stuff. A lot of lead came out of there. Interesting, since I never had a problem shooting the stuff in my 10/22.
When ammo starts out ok from a clean gun and later begins to keyhole, it's because the ammo is leaving deposits in the bore. In my case, cheaper bulk pack ammo wasn't the cause. Lead bullets were. I shoot bulk pack Federal copper jkt'd bullets a lot with no issues.