Pitting in most rimfires is caused by rust. You don't see it very often in regular 22s (non magnums) because the wax, sometimes graphite, outside bullet lubes form a barrier that protects the bore from moisture. This is why you will often find old used 22 rifles/handguns rusted badly on the outside but with still perfect bores. 22 WMRs or 17 HMRs are another matter because neither uses outside bullet lube. Their barrels should be treated as you would a centerfire. On my regular 22s I think it is wise to keep the actions, bolt faces, and chamber areas clean, but usually do a quick pass (bore snake) or nothing for the bores. In the "For what it's worth category" I've been told that Eley, who manufactures some of the most accurate rimfire ammo in the world, rarely clean the bores of their accuracy testing rifles.

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"Speak softly and carry a big stick". Theodore Roosevelt