Could be a couple of things. Chipped extractor/weak extractor spring?
Look REALLY close at the chamber, especially right where the firing pin is. I'm not
too familiar with the auto mag, but a lot of 22's, if they are dry fired, will ding the edge
of the chamber from the firing pin hitting. Case expands into the ding when fired, comes out hard.
IIRC, the AMT automag 22 magnum used a grooved chamber as a delayed blowback
method. Probably borrowed the idea from Hk. While you are looking for firing pin dings,
get a good look at the chamber itself. See if the grooves are mucked up.