spongejosh, neck sizing is really a more advanced technique that I don't recommend to people just starting out reloading. Full length resize until you get experience successfully reloading for your AR.

Then later, if you want, you can experiment with neck sizing (although I think you'll find it unsuccessful in an AR ) but I doubt you'll really notice a gain worth the trouble. Most of the time, people who start recommending neck sizing to others really aren't doing them a service.