I heard from this source, https://www.youtube.com/watch?v=FF0qH_zvfdU, they as good as they get. Do not own one anymore, wish I did, or I would know for sure. BTW, that video may be the most interesting thing I have ever seen on the Carbine.
I've used the Korean mags, both 15rd and 30rd, and they have been fine. I do prefer usgi, though. As for the 30's, my Inland has the M2 mag catch...however, I like the 15's for just about any serious use.
They look good.
I buy a few 10+ round mags any time I read a news story about capacity limitations, and got some Korean 30-rounders but have not used them.
In my experience, even USGI 30s are about a 50/50.