If you travel the gunternet you will, eventually, come across the Korean-made Glock magazines. They are variously called KCI, Kang, and a few other names. The story I heard was that Glock made a sweet deal to the Korean military for a contract and figured they’d make it up on the magazines. The Koreans, as I hear it, surprised them by buying the guns at the promotional price and then said “We’ll take care of the magazines ourselves, thanks for the deal on the guns though.”
True? Probably not. But, it seems plausible enough.
Anyway, for the last decade we’ve seen Korean mags coming in to the US in standard configurations and as happysticks and drums. I went and bought a bucket of .40 mags when they first came in and was severely disappointed. So much so, in fact, that I returned them to the vendor. However, at some point I came into a large (ahem) quantity of the 9mm mags. I stuck them away in an ammo can figuring I’d wait for the next mag ban and unload them then since I had no intention of using them. 
Fast forward a few years and I came into the Ruger 9mm carbine that takes Glock mags. I decided that I really didn’t want to shuffle my existing stash of ‘ready’ mags around so I pulled a couple of the Korean mags out of storage to use with the Ruger. To my surprise…they worked fine. The feed issues I had in .40? Not there. The swollen magazine body sticking int he mag well? Not there. Thinking it might have been the gun, I also tried the Korean mags in my G17 and…worked fine.
I’m not going to say they are something I’d ‘go to war’ with, but as range magazines it looks like they might be a good choice. Although, at this point, they sell (dealer cost) only a buck or two away from a Magpul Glock mag. And I’d rather have the Magpul mag over the Korean mag, and I’d rather have a factory Glock mag over either. But…it looks like the Korean 9mm mags of normal capacity seem to be an okay purchase.
I have several of the 9mm Korean KCI mags. Never had an issue. In fact, I find that they work better right out of the box than the Magpuls, especially on the longer stick mags.
It seems to me that KCI design and quality control has improved significantly in recent years.
I had similar problems years ago and have not had any problems more recently.
I got some on a sale recently for $4.99 each.
Makes sense, the G17 mags were probably perfected for their military, everything else is export junk.
The happystick mags in 9mm have a good history in my circle of friends. And at half the price or less of Glock equivalents. Mostly in 9mm carbine use. I haven’t heard of any of the early 9mm Korean mags ever having any problems, regardless of length fwiw. The 40 issues have been well discussed and I _believe_ have been fixed.
The story I heard was that the Koreans bought the contract for Glocks + the rights to manufacture X number of mags for them. Then when the contract was up, they just used the tooling to make the Korean version we all know and love today.
I have had 4 KCI, 9mm , 15 rd, mags for about 10 years.
Never an issue.
Only used for range use tho.
I had some Korean extended (31 or 33 I don’t recall) mags forever ago and they were utter crap.
Nowadays with Magpul in the 13-16 range I’ll stick with those.
The KCI mags I’ve bought for my M-1 have all been traded off. They just wouldn’t function even once for a full 20 rounds.
Most of the old, used, beat up GI mags I’ve bought have worked.
The ones that hiccup get red taped for range use only if I can’t fix them.