Aside from the USGI 20-rounders, I only have experience with the
Brownells &
Magpul M3s. I have found both of these to perform without a hitch.
During my rare times firing an AR from the bench, only a 20 will do for me.
On walkabout with an AR, only the 20s look/feel "right" to me in the magwell. If I were concerned that someone(s) taking shots at me during my stroll and/or if the AR were selectfire, I would probably prefer 30s (or
more).
On several occasions since the
Brownells 20s appeared (~15 years ago?), when I have found them on-sale, I would buy 10 more. On more occasions I have successfully (barely)
resisted the urge to buy 10 more after refreshing my understanding of just_how_many AR mags I have on-hand.
A couple of years ago I stumbled across some
Magpul M3s on-sale online and purchased 4 or 5. They work very well.
BTW, I have found that the Amazon-brand of "Snack Bags" (
Solimo) are sized almost perfectly for a USGI-formfactor aluminum 20rd AR mag.
I have found them to be handier for
light-duty service (briefly in coat/back pockets, bottom of bags, etc) than an alternative that many here will recognize: