The .22 lr uses a 'heeled' soft lead bullet, with the part of the bullet inside the case smaller than the rest, which is the diameter of the case. LOOK at one and see. The case measures around .223.
The .22 Mag RF has a 'regular' .224 bullet inside a case that measures around .235 at the mouth of a loaded round. So firing a lr in a mag chamber will result in a grossly swollen and perhaps ruptured case, not a good thing in something as lightly bolted as the average rimfire rifle.
I have ACCIDENTALLY fired a .22 lr in the Mag cylinder of my Ruger Single Six convertible revolver. No catastrophe. The case swelled; the bullet hit in the group slightly low. A bore cut for lr only is probably too tight for a magnum, even if rechambered properly. A convertible like my Ruger is bored for the Magnum bullet, and the lr slugs up .001" to fit. Accuracy under these circumstances is surprisingly good, my Ruger shoots a bit tighter with lr's in its lr cylinder than it does with magnums in the WRF cylinder. --slabsides
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If they take our guns, I intend to let my hair grow long and acquire the jawbone of an ass.