I bought a box of 5 King Arms 68rds metal M4 mags and though they wobble side to side a bit, did some test firing with 4 mags worth, no issues yet. Only real problem was that they wouldn't slide out when i hit the mag release but it was just a matter of working them in, I found they slide out better now after doing some quick reloading practices.
"Hey I'm the first one to say its a great country but its a straaaaange culture. This is a place where gun store owners are given a list of stolen credit cards but not a list of CRIMINALS and MANIACS. And now they're thinking of banning toy guns...AND THEY'RE GONNA KEEP THE F**KING REAL ONES!"
-George Carlin 1937 - 2008 (RIP buddy)