+1 for spikes
Spikes upper runs about $549, but for $499 (+tax) I would recommend the new S&W MP 15-22...
The S&W has working bolt catch/release, and all the other normal AR fire controls.( which does offer some advantage over even a dedicated upper). And I've run 500+ rounds through mine in the last week with no problems...
But if you go upper I'm a huge fan of the spikes.