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Posted: 2/16/2011 5:13:54 PM EDT
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Guys,
I have a new build 5.56 AR that is shooting two groups about an inch apart when fired from a magazine. One group is lower and to the right. 5 rounds in the mag gives me two rounds in the upper group and three in the lower. Rifle is a 20" BHW Poly 3 heavy barrel on a BRO upper. Has a YHM Light Rail free floating forearm. Am using the top quality Bravo Company coated BCG. It headspace checked fine. Ammo is 55 gr winchester ball bullets loaded with 23.2 grains of DP2200 on LC07 x1 cases fired by old silver WSR primers. Have tried 55 gr NBTs with 23.0 gr of DP2200 and a load with 69 gr SMKs propelled by BLC-2 (I don't remember the charge weight). The NBTs and SMKs group to between 1/2" to 3/4" in a 20" Bushy HB and a Tikka Varminter bolt rifle. Any thoughts on what could be giving me two groups? (Yes ... I have changed scopes to one known to shot well on an 18" 6.8.) Thanks! Mike |
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If the same ammo, and no rhythm or reason on the different groups (read not consistent with the first few shots one way, then the rest the other way, but happens random during the mag, then either the scope or mount are not holding zero, or the barrel nut is loose, and the barrel is walking/canting a bit in the upper receiver barrel socket, and the shift change.
Scope and mount would be a starting point, and if with a know zero holding of two, then barrel nut would be the next place to look for a problem (loose). |
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Pulled the 5.56 HB rifle with the Black Hole Weaponry apart and squared the upper. Wasn't badly out of square but was a little.
Did that to 4 rifles over the last few days. Noted that an ARP 6.8 bbl had a very good fit to the upper, a WOA 6.8 bbl and a Paladin Machine Services were only slightly looser in their uppers, but the Black Hole Weaponry bbl was quite a bit looser. I bedded all of the barrels with blue loktite. Did make sure everything was tight when they went back together. They are curing for two days. Will see how the thing shoots tomorrow. |
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I had a similar problem when i first put mine together. I used a sabre defense 16" middy barrel, a YHM rifle length FF rail. I shaved down the front sight block. As i would shoot and barrel heated up i would get a second group. Checked everything looked ok. Then decided to shave more from the gas block. After going to the range and shaving gas block a little at a time all is fine now. It seems that the barrel gas block was touching the rail.
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