Can you watch some of these folks load their Glocks at your range (or authorized clearing/loading station, etc)?
No way to know what's happening - or the cause - until you can observe it happening, or experience, for yourself. "He/she said" just doesn't help clarify these sort of things. Kind of like when someone tells you that their gun "jammed".
If it's being user-induced ...
Short version? Often not a "problem" with the gun/mags.
If they're charging/loading the chamber by
not immediately and completely releasing the slides, allowing the slides/barrels to briskly run forward, but instead they're keeping their hands on the slides
long enough to slow/retard the slide run, they might be initially slowing the slides enough to create unintended increased "drag" on the top round. Slower slide travel can create more drag on the top round (while faster slide travel is usually less likely to create as much friction and drag).
If that's what some of them are doing, then the unwanted drag may sometimes displace the occasional top round enough so the bullet nose hangs up on the bottom of the feed ramp, and is then pulled free of the mag lips when the mag is removed to top it off. It "pops" out from under the mag lips.
This sort of thing has been observed and reported among other makes of pistols for many years, in both single stack and double stack magazines. It's been reported by now and again by different folks using different guns in the major LE calibers over the years (meaning 9, .40 & .45).
Oddly enough, increased drag between the slide's pick-up rail and the top round (especially in a full mag) may occur at each end of the mag spring's service life, meaning when brand new (tight pressure), as well as when old and worn (spring can't keep round pressed against lips firmly enough to resist either recoil or pick-rail movement).
The profile of the cartridge can sometimes have an influence, as well. For example, in 9mm pistols, a 147gr JHP (with its longer OAL), if displaced just enough forward by increased drag with the slide's pick-up rail, may be more likely to reach and hang up on the bottom edge of the mag lips, than a shorter OAL round.
This is usually just a "loading the chamber & topping off" issue, though. If the same folks were to load the gun, and
not remove the mags to top off, the slightly displaced rounds would very probably be pushed back by the cycling slide, and then feed normally as the pistol cycled. Some range demonstration might help ease concerns (just chambering/loading from a full mag and shooting without topping off the mag).
I learned this listening to other instructors, and armorer instructors, and experiencing similar "top round" displacement as mags were removed from 3913's, to top them off after chambering a round. Even with the top round initially being displaced (by shooter-induced slow slide run forward), though, the guns would typically feed and function normally.
If you ever browsed the S&W forum you could occasionally read about someone posting a question about seeing this happen, and it concerning them when they saw it, even if it wasn't actually causing a feeding or functioning problem during live-fire. Just looked weird when they saw it.
Now, as the instructor and armorer, it's also prudent to inspect the gun and magazines in order to reasonably to eliminate the potential for any equipment issues (versus user manipulation issues
) to be occurring. Sometimes magazine lip, follower or spring issues may rear the ugly heads, even with the best quality guns. Parts is parts, and magzines are assmeblies of parts, after
This is why I normally liked handling this sort of question at the range, where it could be evaluated and demonstrated during both observed administrative handling
and live-fire. If it's the more commonly user-induced issue, then being able to demonstrate that it won't normally interfere with the normal feeding and operation of the gun (equipment) can be a good thing for the line staff.
Of course, like many other instructors, I've had
at least my fair share of opportunity to have someone demonstrate how
they could manage to induce a resulting feeding/stoppage problem when
they did a "press check".
My answer to them? Don't do it that way. Wanna argue about it? Fine, then it's probably a good time to become better skilled at resolving stoppages ... especially if
you manage to keep causing them.