I just built the same thing kinda. I wanted a fun gun that I could stretch out a little. Ended up going with a Criterion 18" HBAR in 223 Wylde. 1-8" with threads. Looking back on it I wish I would have gone 20" with a target crown. Put a cheap Jerry Miculek style compensator on it, and with earplugs and muffs shooting under the pavilion, it will rattle fillings. Have yet to put a scope on it yet, but was planning on an SFWA fixed 16x but thinking now about going with the vortex strike eagle to give me a little more flexibility.