I don't know the precise answer to your question, but take a look at BATFE Ruling 2011-4, found here: http://www.atf.gov/regulations-rulings/rulings/
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman