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If something is trapped underneath copper it is no longer exposed to the air (H2O). Therefore it shouldn't cause rust since it takes oxygen to cause rust or ferrous oxide (FeO).
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That sounds nice, but many years ago, my chrome-moly varmint rifle barrel was ruined because I thought it shot better from a fouled bore and left it in a closet, leaning against a metal sewer vent stack, which must have condensed moisture on it during the winter. When I looked at it in the spring, I could barely see through it! It was badly pitted and it took almost forever for solvent to saturate the rust enough to get a cleaning rod through it. (There was NO rust on the outside surfaces of the rifle, which were protected with Rig.)
Funny thing, though; despite being very pitted, it still shot very well (5/8" prone group at 200 yards). I sold that rifle because I couldn't stand having such a pitted barrel on a rifle.
The key is not that oxygen as a gas can get under the copper, but its the moisture (H20) that can provide enough oxygen to cause rust, and will seep under the copper in various locations, probably carrying firing contaminants to create electrolytic corrosion. Regardless, centerfire chrome moly barrels should be cleaned as soon as reasonably possible after firing.
Twenty-two LR rimfire barrels are not subjected to the same conditions as centerfire barrels, due to the wax applied to them every shot. There are
no .22LR bullets manufactured that do not have a wax coating. Some bullet waxes are harder than others, but they are all coated. That's why it's generally safer to buy a used .22LR rifle that hasn't been cleaned than a centerfire with a dirty bore. (Regardless, I like to run a patch through all used bores before buying.)