those are caused by bad magazines, when they happen on fresh rounds. it's the same problem as TRUE "double feeds" (not the commonly mis-represented "double-feed" with one fired, one fresh round- which is NOT a double feed). the magazine feed lips aren't retaining the top round, so when the bolt rides back, a round pops out and subsequently jams up the action in one of two ways.
ETA - i should clarify, WHEN it's a chronic problem. if it happens once every few thousand rounds, that's probably just a fluke. but mark the magazine that commits the sin, so that you can chart patterns. if a magazine get more than 1 mark, it goes in the trash, with my kit.
ETA - i should clarify, WHEN it's a chronic problem. if it happens once every few thousand rounds, that's probably just a fluke. but mark the magazine that commits the sin, so that you can chart patterns. if a magazine get more than 1 mark, it goes in the trash, with my kit.