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Posted: 6/6/2011 5:05:02 AM EDT
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Hey Guys,
I just finished building my first AR. Its a .204 Ruger setup for prairie dogs. Everything works great and I found a load that shoots about 3/4" groups. The only problem I am seeing is that as the barrel heats up, it starts to string shots vertically. My first groups from a cold barrel are round, or slightly horizontal, but as the barrel gets hot, the groups turn to vertical lines and open up to about 1.5". Of course its a freefloat handguard, but are there any solutions that you AR gurus know about to tighten this up? Thanks! |
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Thanks for the response. Im not positive that the stringing isnt caused by my shooting, but I'm pretty confindent. I was shooting some other rifles along with this one and wasnt having any issues with them. Also, the stright vertical lines are making me think something around the barrel connection is changing as it heats up. Typically when I start shooting poorly, the groups just open up in all directions. So here's a little more information.
From a cold, clean barrel, this rifle shoots about 3/4" groups, that are typically round or slighlty horizontal. As the barrel got hotter, the groups becuase straight vertical lines, and they were around 1.5" long. The barrel is a DPMS .920" fluted stainless barrel, so I cannot use a heavier contour. The handguard is free floating, but I did have to torque it to almost 80 lb-ft to get the barrel nut aligned. I've heard that accuracy is best closest to 30lb-ft, but I couldnt get the barrel nut aligned. So my question is has anybody had a rifle string shots like this before, and how did you correct it? Would lapping the Upper Receiver face help? This might correct any out-of-square issues on the face of the reciever, and I could also use a much lower torque to align the barrel nut. It could also be possible that my ammo had heated up somewhat and is the problem, but the same powder is used in my other rifle and it doesnt seem to be a problem. Any advice would be great. |
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I had a vertical stringing issue as the barrel heated up on my M&P15T. It was worse than yours, and I could not work out what was going on; checked, loosened and re-tightened everything and was still having trouble. Did a lot of asking around and googling around and I was advised by some extremely proficient shooters/builders that using blue loctite on the barrel extension and the face where the extension mates into into the receiver was the thing to do. And whatdya know....it worked, and cured the stringing issue.
What this does is effectively bed the barrel to the receiver. If your barrel/receiver is somewhat of a loose fit, the only thing that is holding it all together is the front face of the receiver on the barrel, and the nut torque. As it all heats up, things can shift minutely behind that loaded front face and cause the stringing issue - (and I have heard this can be exacerbated by heavy barrels such as yours, placing extra weight and stress on that area of an aluminum receiver not originally designed for big heavy barrels.) When you put the loctite in there (or a high temp RTV compound will also work), it acts to spread the load across your entire barrel extension, rather than just the front face, and effectively gives it the fit of a very tight barrel extension/receiver. Might not be your issue....but for what it costs in time and money, you have nothing to lose and everything to gain and I now do it as a matter of course with every AR I build. Pains me to do this, but I have to put some footnotes in before uneducated masses chime in and completely rubbish the idea as sometimes happens: 1. This is not about putting loctite on your barrel nut threads. Put moly grease on them as usual; wipe off any excess loctite that gets on the threads and the inside of the barrel extension. 2. Loctite does not melt. Heat breaks down its shear strength but it never "melts and becomes runny", and never "gets in your barrel extension and chamber", and never "flakes off" either. In this application, it will continue to serve as a gasket/filler compound until you take the barrel off, even if heat does completely break it down - which it won't, because the majority of the heat is produced forward of the barrel extension. 3. Loctite, especially blue loctite, is not some mythical compound that you will never be able to take apart, ever. All you have to do come barrel replacement time is put your barrel in a vise, and using a rubber mallet gently tap the upper receiver backwards off the barrel. You can then use brake cleaner and a soft scraper to remove the old locitie off the inner receiver/barrel extension, ready for reuse. Simple as that. Here's some more good info on accurizing your AR - pay particular attention to the posts by poster RCW3, who has forgotten more about AR accuracy than most will ever know. The tip above, and your comment about truing up the receiver face are both in there, amongst other things: AR accurizing |
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It's for .204 ruger and the stringing is 1.5 -2 inches straight up? That is suspicious. It should string diagonal if it is heat.
Please before you do any mods check your scope rings and loc-tight all the mounts. Check the free float barrel nut and the upper's engagement face. The vertical nature of this, is wiggle somewhere.... |
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