Additionally, this video shows a slower-cycling 9mm AR using factory Glock magazines to churn through 5000+ rounds on full-auto without too much fuss, at least as far as feed rate is concerned. See this videoof a Glock 18, where at about 1:40 Larry Vickers warns viewers that 33-round Glock magazines often experience stoppages around halfway through a full-auto mag dump. Reduced maximum feed rate is typically not an issue for civilian semi-automatics, which are usually kept clean and only asked to feed a single round per trigger pull, but for quick-cycling full-auto weapons used out in the wild, it can be a problem. Rounds tend to bind up in the transition from double-column in the lower magazine to single-column at the top for feeding, especially at high cyclic rates or when dust/debris gets inside the magazine's already-crowded interior (hence the low-friction PVD magazines with internal sand groove that became necessary once the similarly-designed Beretta M9/92 magazines started seeing extensive desert use). Single-column feeding magazines are less dirt-tolerant and have a lower feed rate.Therefore, a single-column feeding magazine allows for a slim, manageably-sized handgun, but it also comes with unavoidable design compromises: The feed ramp for a double-column feeding magazine needs to be roughly as wide as the magazine feed lips, and while there’s plenty of room in a submachine gun’s breech, the same is not true of a handgun. The reason for this is simple: double-column feeding is better, but single-column feeding takes up a lot less space. Virtually all double-stack pistol magazines use single-column feeding (the FN Five-SeveN and Russian Gsh-18 are two rare exceptions) and virtually all submachine guns use double-column feeding (the H&K UMP-45 and the MAC-10 are two rare exceptions).

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