Redi-mags, Redimags, Redi-Mods - all the names for future search purposes.
Once upon a time, they were all the rage for carrying a spare mag on the gun. No ancillary gear required. But they seem to have disappeared from the marketplace.
I still have one floating around. Was thinking about it from the PDW/truck gun/compact AR pistol perspective. Does it make any sense? being able to have 60 rounds on the gun with nothing else to grab or carry is appealing. The bulk and manual of arms are the obvious drawbacks. I suppose they're in the way if you have a LAW folder, too (why does a LAW fold to that side, anyway?).
Anyone still a user of these?