I had C products years ago when I had mine. I also have heard that ACS were ok but did not get to try them out.
a 5.56 20 round straight mag will work... just can't get 20 rounds in. if you can find them, the 'frankenmags' worked well. it was a AK mag with an AR mag fixed to the top... thus the Frankenstein reference. lastly, you could get a lower made by MGI which has the ability to swap out the magazine well for; AR-15, AK-47, greasegun (45 cal), 9mm colts mags and a bunch of others. The mag wells aren't serialized so with one lower you can have a bunch of uppers and the proper magazines to fit. They're expensive though. https://www.mgi-military.com/store/index.php?category=9
I am looking for a legit 30 round mag. I'll do some research on the ACS's and the C products. Thanks.
There are several AR's that accept AK mags. Rock River builds one but I think the better one is made by CMMG. Their lower must be used with their upper which uses a AR10 BCG so you basically only can buy a built rifle but it seems like the thing to do if if really want to run 7.62 x 39 in a AR http://www.cmmginc.com/product_p/76afc41.htm http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=558
I haven't looked for any for a couple of years now but if I remember correctly i was always able to pick up 1 or 2 at a local gun show. You should be able to get them from the intermess too. Shouldn't be too hard to come by, you may not like the prices though.