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Posted: 1/29/2014 6:46:08 AM EDT
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A friend of mine, who has built a couple of accurate AR's successfully, recently built a rifle with a Brownells 16" mid gas barrel for his son. He used barrel: http://www.brownells.com/rifle-parts/barrel-parts/rifle-barrels/ar15-m16-308ar-barrels-prod56797.aspx in 1-7 twist 5.56 medium contour. Since building the rifle he hasn't been able to get anything close to acceptable accuracy out of it. He has a Nikon 223 scope and mount on it, premium BCG, free float HG, Magpul accessories and stock. The barrel torqued in at about 50#.
The best he could do initially is about 3 MOA with occasional flyers 4 or more MOA where ever it decided to throw them. He figured it was a scope problem so he called Nikon since the shots were all over the place and adjusting the zero really didn't result on consistent POI changes. Nikon sent him another scope and we installed it and properly torque it. We went to the range yesterday with his Lead Sled and the rifle. We had the same and even worse results. 3-4 MOA with flyers. We tried every combination of ammo we had loaded at various velocities and weights. 55gr V-Max at 2750, 2800 and 3000FPS using BL-C2 and Varget. 62gr match bullets at up to 2800 using BL-C2. 69gr bullets, 80gr bullets all at various velocities and OAL's. The best we could do was 3 MOA for 3-5 shots with the V-Max at 2750 and Varget and even that was no less than 3 MOA with unexpected POI shifts along the way. All of us shot that rifle and others and all of us were able to pick up any other rifle we had and shoot them at 1 to sub MOA so it wasn't the shooters. I'm thinking he has a defective barrel since there's no other indication of problems with the rifle. It functions perfectly, throws all the brass at 4 o'clock with no swipes or pressure signs. A perfectly functioning rifle that shoots shotgun patterns at 100 yards. I have never seen a 5.56 AR shoot this badly in my life. Any other ides about this from you guys? We've had it apart and everything is tight, lined up and secure just as it should be. He's going to call Brownells in the next day or so so any suggestions would be greatly appreciated. |
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Calling Brownells is the thing to do. Anything you do to it now would just void any warranty but I suspect that there is either a chamber problem or a muzzle crown problem.
Brownells will stand behind it I am sure, but if not, I would recrown the muzzle with a ¼ inch round head brass bolt (BRASS, not steel) and some 600 grit lapping compound with the bolt chucked in a drill motor. Lap with plenty of lapping compound and stop when you have a nice lapped ring all the way around the junction of the bore and the muzzle, i.e. the crown. I have turned a couple of sour shooters in real shooters by lapping the muzzle, but in each one of mine you could physically look at the crown and see damage. If the chamber is wallowed out or not on the center line then the barrel is toast and will have to be replaced. |
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Crown...Duh! I can't believe I didn't think of that. He called Brownells and they told him they would take care of him. The Tech he talked to told him he should expect MOA to sub MOA out of that barrel unless it has some sort of defect. He took the flash hider off this evening and sent me a picture. If what I see is what I think I see the crown is a POC. He says he sees one land higher than the others inside the muzzle and the crown looks like it's unfinished, rough and uneven. He said he honestly didn't look at it carefully when he assembled the rifle. Brain fart he said. He's bringing it up to show me tomorrow morning because I just want to see it for myself. It will be going back to Brownells regardless. I'm especially interested in what he sees as a tall land. He says the rest of the rifling looks normally faint but one is obviously higher. Boy, that would certainly produce exactly what the rifle is doing. I had the same issue with a dented crown on an M1 Garand that protruded into one edge of muzzle. It would group then a flyer then group somewhere else...all over the place. I had that one re-crowned and it was one of my best shooters afterward.
Thanks for the heads up on the crown. At least it may have pointed us in the right direction. |
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I had a very similar problem with my AR, which has a BCM barrel. It turned out the problem was that the bullets were hitting the flash hider upon leaving the barrel. You could see very faint impact marks on the inside of the flash hider. You might try taking a look at your flash hider and just try replacing that, since it is only like a $10 part.
Here is the link to the thread where my problem got resolved: http://www.ar15.com/forums/t_3_4/622851_.html |
| It could be that the barrel is out of spec with the bolt. Excessive headspace will cause poor accuracy as will an out of spec chamber, defective crowning or barrel threads that are not perfectly concentric with the bore. Note that Brownells is just a middleman that doesn't manufacture anything so it is possible that they can get defective products every now and then. If its the barrels fault Brownells should make good but if its headspace that means trying different bolts until the proper headspace is achieved. |
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Update on this. I went down to look at the barrel after he took the flash hider off and cleaned things up. It's a 5 groove barrel and the crown itself looked good. But, the real problem was very obvious when you took a look at the rifling right at the crown. Four of the lands are perfect but one has a chip missing on the end of the land and a deep scratch that extends at an angle across the land about 1/16" from the end of the barrel. You can catch the scratch with a dental pick and the last 1/32" of the rifling groove is actually missing from the end. The barrel is going back for certain. Since the barrel came with a cap over the crown and threads it's unknown how that could have possibly happened unless it was a mistake at the factory or a flaw in the steel. Stranger things have happened. A lesson learned here for me. Always inspect the rifling at the crown of the barrel before you install it to make sure the rifling is clean of damage or defects. He said he never even looked.
Larry |
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