AR Sponsor
Posted: 9/2/2014 4:46:39 PM EDT
|
Hi,
I had a 6.8 spc upper assembled by a well-known, top-notch custom shop. This is the 3rd 6.8 upper that this individual has built for me: 2 are 6.8 SBRs and are less than .5MOA guns, but the third groups like crap. The upper was assembled with the following new components: Vltor MUR upper receiver, Young Mfg bolt carrier, LMT 6.8 bolt, Noveske 16" light contour barrel, and BCM KMR fore end, with exterior parts professionally Cerakoted in FDE. The gentleman that built the upper has been great to deal with. We both thought that the barrel was the issue, although he has had only seen one or two Noveske barrels that wouldn't group well out of hundreds. (Lower and optics/optic mounts have been ruled-out as causes). We got Noveske involved and they actually had a new barrel Cerakoted to match the original, and installed the new replacement barrel themselves--the assembly by both parties shouldn't be an issue. I got the upper with the replacement barrel back from Noveske last week and took it to the range over the weekend. It groups worse than the original: about 2" at 50 yards. (The guys at Noveske couldn't get the first barrel to group better than about 2 MOA). The first barrel wouldn't group well with any commercial (SSA, Hornady) load, or with any hand load--about 6 or 7 different types of bullets tried ranging from 85-115g and as many appropriate powders as could get my hands on. Any ideas on a possible cause? One bad barrel I can buy, but two????? Anybody had accuracy problems associated with the BCM KMR. Thanks! |
|
Its possible to get two bad barrels, but would suggest pulling the FS off the barrel and shooting some groups to see if its the problem instead. At the same time, will allow you to give the barrel a good Sweets solvent cleaning, and even take a look at the throate and crown for any burs that may need to be shot out via live fire removed during stress relieving.
Also, what is your break in time with the barrel to allow it to stress relieve/unify the rifling, and are you reloading ammo specific to the barrel? Next to point out, you do have a light weight barrel, so heat is going to be a issue if you are rapid firing through the barrel for groups. Lastly, hand charge a round through the rifle, carefully extract the round and start checking it for run out. Could be bad feeding through the rifle that is canting the bullet in the case, and the less than perfect rifle embedding to the lands at live fire is causing the problems with the bullet not sleeping fast enough for the range that you are shooting. |
|
I really appreciate the response.
In the sentence " pulling the FS off the barrel and shooting some groups to see if its the problem instead," what do you mean by "FS." Fore-end stock/hand guard? Yes, working up loads specifically for this upper. (I've been handloading for a decade). Nothing grouped well. All of my other Noveske uppers have been sub-MOA from day one. I realize that the groups may improve with break-in, but I've never had a gun that shoots so poorly NIB. My SKS groups better at 50. "Next to point out, you do have a light weight barrel, so heat is going to be a issue if you are rapid firing through the barrel for groups." Slowly fired groups. "Lastly, hand charge a round through the rifle, carefully extract the round and start checking it for run out. Could be bad feeding through the rifle that is canting the bullet in the case, and the less than perfect rifle embedding to the lands at live fire is causing the problems with the bullet not sleeping fast enough for the range that you are shooting." What faulty component would cause bad feeding to occur? Stripped upper, feed ramps, etc? Thanks for your advice and knowledge. |
|
FS =Flash suppressor or brake.
Also, with a thin barrel, really should free float the hand guard system, since it just too easy to induce flex to the barrel when the guards are attached to the barrel via the front guard clip. In causes like this where the front hard guard system is attached to the barrel, then sand bag under the hand guards as close to the receiver as you can to fire groups. Granted you will still flex the barrel a touch, but not as bad as if you have the sand bags all the way out towards the front hand guard clip instead. |
|
Thanks. Don't know why I brain-farted on "FS" being flash suppressor. Probably because I've been fixated on the hand guard. I did try shooting groups with the muzzle device off with the first barrel: No improvement at all.
The hand guard is free floated. I did notice that the bottom of the gas block rests .08" above the innermost portion of the hand guard. I don't know if there would be enough barrel vibration on firing for there to be contact, and, if there was, how much this would affect accuracy. |
AR Sponsor