Do some research and build one...

I'd recommend you get a stripped lower and buy a parts kit (I'm sure there would be more than a few folks that would help you out here), then buy a complete upper. It will be headspaced and test fired from the factory. Attach upper to lower and blast away.

I'd make sure you got one chambered for 5.56mm and a 1:7 to 1:9 barrel twist. That way it will stabilize the heavier bullets.

I've bought a few AR's complete in the past... built my first a few years ago. The nice thing is you get exactly what you want. What I wanted was a "KISS" lightweight A2 with a mid-length gas system. Here it is:




Ask a bunch of questions along the way... good luck.