100yds is a good distance to zero your rifle.

Competition, you would learn a lot simply by coming and watching a match.

http://www.ar-15.co/forums/showthread.php?t=37424

Suggest shooting the best ammo you can, such as Federal Gold Medal Match 175 grain.

How were you shooting? Prone? Bi-pod? Rear bag?

The last time I was here, the owner told me he planned on starting long range rifle classes soon, give them a call, good bunch of guys.

http://www.triggertimegunclub.com/