I have tried the 15 round magazine, and I believe it was not a problem at least with the new one, it seemed to be a problem with the old ones that came with it. (I only tried it once though)

These were the new mags I purchased:
KCI-MZ026 KCI M1icon .30 Carbine 30-Round Magazine
KCI-MZ025 KCI M1 .30 Carbine 15-Round Magazine

I have only used purchased ammunition, thought I don't know the brands off the top of my head. I used two brands, I think I had misfires from both. 3 out of 45 rounds. All three rounds were able to fire when I reloaded them and shoot them again. I know that if you don't let the bolt slide back on its own when you rack it back it will not fire either. I am wondering if it is not getting enough blow back to throw itself forward all the way. Sorry don't know all the lingo.

Any more information you need? I can provide actual pictures of the gun as well. Or answer anymore specific questions.