Team Hoser doesn't split hairs on what mags can or cannot be use because in the overall scheme of things.... It doesn't seem to matter.
At the risk of sounding like a broken record...or these days a corrupt mp3 file, it doesn't matter what kind of mags you use if they're not 100% reliable.
Also, The few extra rounds you have don't give any advantage when you don't know when to change 'em during a CoF.
I've seen a guy using pinned 5 round Pmags shoot a CoF where when he wasn't shooting or when he was moving he'd change mags... Next fella using his Beowolf's just kept shooting until he ran dry.. Then stood there squeezing the hell outta his trigger wondering why no more bang bang?
Then when he finally realizes he's empty the struggle to change mags begins. Once the empty mag is out and the fresh one in there that loud "click" as the hammer falls and nothing happens... Again, a pregnant pause as the shooter comes to terms with having not chambered a round...
Anyway.... It's the guy with the pinned 5 rounders who wins because he 1. planned his CoF and 2. learned himself how/when to change mags.
As far as the box fed shotguns go and "open".... Here in Canada I've never seen one that actually functioned reliably. The poor guys who use 'em spend more time dickin' around with 'em than shooting. Maybe it's a case of "cool gear no practice", but they're the slowest gun on the range.