The zytel mags that sportsmansguide.com sells are the cobray brand. It works way way better than the crap metal 30rnd mag I got from cheaperthandirt.com.
At first the Cobray zytel wouldn't feed correctly, it just took a mine amount of sanding on the lips. Now it works pretty good, I get a failure to feed about 2 to 5 times from the 30rnd cobray. I notice the more I shoot it, the better it is feeding(less failure to feeds). I think once it wears in completely its going to work flawlessly.