18 USC 924(a)(1)(B) and (D) create a maximum five year prison sentence for a violation of 18 USC 922(w). Basicly you'd be looking at a felony conviction with a maximum of 5 years in the cooler. Even if they let you off with less than a years time, you would still have a felony conviction on your record, loose your right to ever legally own a gun again, loose your right to vote, and spend a truckload of money on an attorney. And yeah,you'll hear the guys who say they've never heard of a case whre someone was convicted on this, but I'd be curious how they track this issue. What they don't know is how many people may have been charged, or how many of them may have copped a plea to a slightly lesser offense, pleaded nolo contrendre or guilty, or had a case which simply wasn't reported because it was such a no-brainer. And Sparky315 has a good point; just because you haven't heard of any convictions, doesn't mean you won't be the one they decide to make an example out of.
Your chances of getting caught are probably pretty slim, but IMHO its a huge risk to take to save a couple bucks on a few mags.