Building AR's can be addictive. I had built a 300 blackout rifle a few years ago using a 16" barrel and a polymer lower. This pistol I used an 80% lower (my first) fitted to this upper. I was not sure how the gun would run. I ran just fine. It also did not take time to sight in. I've put roughly 120 rounds through it thus far. No complaints. Radical is now my go to upper.