Can anyone tell me what the difference is between a magazine for an ar15 with a 7.62 x 39 upper as compared to a stock magazine for a  ar15 .223.because of the angle of the 7.62 x 39 shell me and my brother figured there had to be some difference in the magazines.we were at a gun show the other day and seen a stock 20rd colt magazine that said on the floor plate that it was for 7.62 x 39,but we compared it to a stock 20rd magazine for the .223 and we could not tell any difference in any of the dimensions or the followers between the two magazines.and my brother said he put some 7.62 x 39 shells in a stock .223 magazine and they just did not seem to load right inside of it and were kind of plopping around inside of it.can anybody help me out here?........thanks in advance.

In the spirit of brotherhood:




http://www.ar15armory.com/forums/Review-cProducts-762x3-t33659.html



I made it cold.
 

The magazine that looked like a standard 20 rounder may have been a Colt 5 round magazine. You can use a standard 5.56 20 rounder with 5 or 6 rounds of 7.62x39 loaded.

For greater capacity the 7.62x39 round requires greater curvature in the magazine by virtue of the shape of the cartridge. You can see this just by looking at some AK mags. Also, since the round is thicker than a .223/5.56 round, dedicated 7.62x39 magazines (C Products on the right below) require the ridges on the sides to be not as deep as 5.56 magazines, to accomodate that fatter 7.62 casing:

IIRC, Colt used a blocked 20 round mag with a different follower.  Remove the block and it still won't feed all the rounds you can stuff into it.