Project Appleseed

I would agree with using 20 round mags.  I have also used 10 round P-Mags.  If you are concerned about extracting a sticky magazine, with P-Mags you can add a Magpul device to facilitate extraction.

If you use 30 round mags make sure that the bottom of the magazine remains well clear of the ground in your prone position.  You are not allowed to use the magazine as a monopod support when shooting prone in Appleseed.

It sounds like you have some solid advice so far — the Frontier 55 grain is fine, and you should get another 20 rounder. Bring the 30 round to the event as well. It's good to have a backup. And you can use it in standing. I like to have some extra magazines I can load ahead of an AQT, so you could put 10 in that one in advance for the standing stage and get a bit ahead of things.

Better to have and not need than need and not have. They say that all the time. That's why I own far too many magazines. Well, and I just like buying stuff too. :shrug: