If you're going to block the mag, don't block it at 5 - but at 7.
In my opinion, just get a 5rd mag (or two). That's what I did - I got them from DPMS back when they were on sale a year ago or so.
5-rd mags make for less hassle to deal with if you happen to pass by a game warden - its easy to look and see that someone has only a 5rd mag in their AR than seeing the 30rd mag and making them take out the mag and check it for capacity.
agreed , i think the extra DOE on an extra mag would save a lot of hassle, and like he said if your gonna block it do it at 7rnds