9mm AR's are allot of fun and cheaper to shoot, I love mine!
If you build one use Spikes 9mm AR parts except for the bolt, use a cmmg ramped bolt so you can use a standard AR hammer.
They are a lot of fun. I recommend the ramped bolt and colt/metalform style mag block.
If I were to do it again, I would just get one of these: Lone Wolf Distributors - Products - Glock Magazine AR-15