Extractor problems are rare with factory parts, but I've seen a few with custom extractors or custom bolts with non-standard extractor cutouts. Make sure the extractor tension isn't too high, or causing interfering with the case rim as the round slides up the bolt face. If the extractor is too tight to move when the bolt is fully closed, extending too far into the bolt face cutout, or binding on the rims at all during chambering, the base of the round will stop before the round is fully level, which will prevent full chambering. I'd recommend carefully examining 5 rounds to make sure the cases and bullets are clean, not damaged, and aligned. Try to fire these five rounds, and if any fail to chamber, carefully extract them and look for any case or bullet damage.
If there are any burrs on the side of the case rim, it is probably an extractor, or extractor cutout/notch problem (binding, burr, or a too-sharp edge somewhere). A sharp corner-shaped punch-mark on the side of the case near the rim may indicate the extractor hook is too long. Any damage on the side of the case (like a curved "scalp" mark, for instance) might point to a problem with the chamber mouth. On the heavy, long-bulleted 60-grain rounds, this type of damage might extend all the way to the bottom edge of the bullet. If the nose of the bullet has any damage, it may be a magazine problem (presenting to chamber at the wrong angle), a sharp edge on the chamber, or even firing pin damage from dry-firing (burr on the edge of the chamber).
Another thing that can interfere with the base of the case sliding up the bolt face during chambering is the firing pin. If it is sticking out of the bolt face during feeding/chambering, it may interfere with the case moving fully into position on the bolt face. Make sure the firing pin is retracting after firing (should be spring-loaded to do so); any burrs or binding that might cause it to stick in the forward position, even briefly, could cause problems like you are describing. If you see a dent on the outer edge of the rim after the firing test, above, it may be a firing pin interference problem.
Because you experienced similar problems after reinstalling the factory barrel, I am leaning toward a bolt-related (extractor or firing pin) problem. You did the right thing in checking the magazine, but as you said, it's unlike to be the sole cause of your difficulties (although a bad or out-of-spec mag may contribute to a problem; I always use several different mags during function testing, and mark them (#1, #2, #3, etc.) so I can trace stoppages to certain mags, if it comes to that).
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Last edited by DJ Niner; 05-20-2013 at 22:54..