I had a lower and was installing a new upper and a M4 barrel. I decided while I was putting it together I would use the FIRSH. I had to remove the front sight post and the handguard keeper. All went as plan. I then tightened down the barrel nut using never seize. The nut to 30lbs three times then tightened down to approx. 72-76 lbs when the gas tube hole lined up perfectly. I tested it with an extra gas tube and went in and out of the barrel nut and upper with no binding or touching. So far so good. I then screwed the FIRSH onto the barrel nut. The lock down screw ended up at the 9:00 position (if looking down the barrel from the muzzle). I then had to back it off, so the lock down screw would be at 6:00. After I back it off there was a small gap in between the upper and the FIRSH. I had a heck of a time lining the upper and the FIRSH rail. The other problem was no matter how tight I made the lock down screw the FIRSH would come loose. The only way to stop it was I used red loc-tite on the out side of the barrel nut and in the inside of the FIRSH threads and screwed it on then screwed it off then screwed on so the lock down screw was at the 6:00 position. I also tightened down the lock down screw with red loc-tite. When I placed my reflex sight on the upper the rail went over onto the FIRSH, but it locks down to the upper. It's very ridged. I went out and put 200 rds through nonstop. The temp was 98 degrees and I left the rifle out in the sun all day and never had a problem with it. I shot it all day long and nothing came loose. Except a little never seize came out of the barrel nut. I replaced the front sight post with an allen screw type. I will try to post a picture of it if I can. The only problem is the front muzzel device. If it's permently attached that could be a problem. I used a pre-ban barrel and converted it to a post-ban it's easy and gives you way more options.