I tested 5 of the Korean 27 or 28 round mags. Not one went without problems... ranging from falling out of the G21 SF after each shot or massive FTF's.
Many could probably be fixed with a little... or a lot of TLC. I did use a razor knife to define the edge of the retention slot on the one, but left the others alone and re-sold them. The best one went 23 rds before 1 FTF and then finished up. The worst had many FTFs.
There's ONE good answer to this: I just got a bunch of the Kriss magazine Extension Kits made by Magpul. I put several together and tested them at the range. They work great! I may actually use the Mech Tech Carbine Conversion that I assembled and have tried to sell becuase I couldn't shoot it at the range I belong to... only long guns allowed are .22's. They're about to finish a 25-yard range that can handle rifles, so that and a reliable high-capacity magazine for the G21 SF-based carbine may make it a keeper for me.
I've since put together 5 more Kriss 30-round mags for resale at the gun show this weekend in Jacksonville at the Prime Osborn Convention Center. My wife and I will be vending there and we have a lot of Glock mags... just about every one made... standard and hi-cap, as well as the new Kriss conversion Kits.
Pix in this post at post# 6: http://www.glocktalk.com/forums/show....php?t=1533219
I don't know if there's a venue for this kind of post, but if I've broken any rules, monitors please put this where it belongs or delete it.
It's not my intent to answer posts on the kits' availability or try to sell them here... just information. I have to get ready for the show and go set up our table this afternoon.
G21 Sf, G23C (Gen3), G19 (Gen4), and G21 SF-based Mech Tech Carbine Conversion