I bought a half-dozen of the 15-round KCI M1 carbine magazines a year or two ago; when they hit the market and were crazy cheap - something like $4-$6 apiece. They seem well-made but functionally they're not as good as the GI ones I have, and the GI ones are all probably 40-60 years old, stored mostly NIW for most of that time. The six KCI's that I have all tend to nose-dive or (on rare occasions) just fail to get the round up to the bolt quick enough, so the bolt just slams home empty. Happens with the KCI's in both my guns (both are GI surplus, not commercial guns), and it never happens with any of my old gi magazines; so I have to leave the blame with the KCI's. Their springs are noticeably weaker than the other mags, and it could well be that just replacing the springs with new ones could solve the problem. I just never bothered with doing so. For plinking and range use, mine are 'just barely' ok; but I'd absolutely not trust them for serious use.
Never tried their 30-rounders.