I carried my Ruger 10/22 to an outdoor range with my son last year, where we plinked steel targets in a shooting gallery to the tune of around 300 rounds in an hour, using mostly Remington Thunderbolt Lead Round Nose in a bulk box. It was fairly clean already... maybe not pristine, but pretty clean.
I never had too much of an issue with these rounds excessively fouling in this rifle before, at least by 22 LR standards, but this time when I got the rifle home and tried to run my carbon fiber cleaning rod through, from the muzzle, with a slotted tip and patch cloth, it ran into something hard and wouldn't budge. So I removed the tip and just tried to run the rod through with nothing, and it still couldn't make it through. The barrel was partially obstructed near the chamber. I tried an Otis cable from the breech and couldn't get that through either.
I fiddled around with this thing... sticking qtips soaked in Kroil and Hoppes #9 into the chamber, trying to loosen it up. Finally I managed to get a bronze brush inserted threads-first into the chamber and past the obstruction, by tapping it in with a hammer, then screwed my rod onto it from the muzzle, and managed to pull it through... a decent sized chunk of lead came out, shiny and rounded in the shape of the barrel on the outer surface. I guess it started fouling, and then every lead round was laying down additional layers as it squeezed past the fouling.
I considered us lucky that we didn't blow the gun up with an obstructed barrel that day, and I completely avoid cheap lead round nose .22LR now... I try to stick with the copper CCI Mini Mags now.