AR accuracy fix

[Weg]

I figured I would share a story about an AR build that was giving me fits, and how I fixed it. I recently got into building AR's and was on my 3rd one when I ran into a major accuracy issue. I had bought a 16" new M4 upper that already had the barrel installed. The upper would shoot about 3" groups at 50 yards with M855 ball with a properly mounted scope. I have never had one shoot this bad, in fact, of the 2 I had built previously , each shot slightly under 1" at 50 yards with he same ammo. I pulled the hand-guards and gas tube and checked the barrel torque. It appeared to be around 50 ft/lbs which was in spec. I went ahead and pulled the barrel from the receiver and still found nothing that looked odd. I found a You tube video about upper receiver lapping, which I had never heard of. I went ahead and ordered a $30.00 AR upper lapping tool from Midway figuring I had nothing to lose. I lapped the receiver Friday, re-installed the barrel and test fired. What an amazing difference! The exact same barrel/receiver assembly is now shooting 3/4 - 5 shot groups at 50 yards. Just thought I would share in case someone else runs into the same issue.

[TexasJohnBoy]

I'm not familiar with lapping an upper, which area was smoothed out? Do you have any pictures? The only lapping I've ever done was on the bottom of heatsinks and water blocks for computers...

[jb2012]

Absolutely! this is sometimes called blueprinting. I know civ-tac does this with their builds.

I'm not familiar with lapping an upper, which area was smoothed out? Do you have any pictures? The only lapping I've ever done was on the bottom of heatsinks and water blocks for computers...

Basically the face of the front of the upper receiver is machined to make it sit exactly straight, and exactly perpendicular to the receiver. This allows the barrel to sit exactly straight and parallel with the gun. Without doing this, cheap machining could lead to your barrel being slightly crooked in relation to the upper receiver.

[Weg]

Actually, the video that JB2012 shared is the exact one I looked at, forgot to add that I did the blue loctite deal also. I'll be doing it on every one I set up from now on.

[TexasJohnBoy]

Interesting, thanks for sharing guys.
On a separate note, I'm glad I don't live in CA.

[Rvrrat14]

I true all receivers, then I know they are GTG. 50 ft/lbs is too much. 30 is ideal. Free float barrel with quality handguard and bed barrel.........Sub MOA at 100 in my 6.8's.