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Posted: 5/26/2010 5:49:28 AM EDT
| OK, so I take the LR 308B out for a spin for 3 more tests. Brand new Winchester cases, Tac powder, and the Hornady 168gr Match BTHPs. 10 round groups at a half inch square. Once again, locked in a lead sled. Once again, the gun is inconsistent. At times, for 4 or 5 rounds, the gun will hammer what you aim at, but then throw off shots. The first test, I hit the bottom right corner of the square, then down 3” from that, then up 4” from that shot. The last test, I put 6 bullets almost in the square, but the gun scattered 4 of 'em. I made no attempt to adjust the scope. This gun has me baffled. You can see the tests here:Targets The targets are the bottom 3 on the page. Click on a pic to make it bigger, then click on original at the top to blow that up. Any ideas? |
| That happened on another gun I tested that day. Groups suddenly went haywire. The riser for the scope mounts had come loose, so we checked this gun to make sure it was tight. But that AR's oddities is another story. This gun was sent to DPMS for repair. I don't know for sure, but I think it's a different barrel. It originally wouldn't shoot for crap when I first got it. Chamber had heavy tooling marks in it and now that's gone. But polishing the chamber wouldn't have shrunk the groups the gun originally was shooting. They did test fire it for sure before they sent it back. Powder residue all over the bolt and copper was in the barrel. It's only had one hundred rounds through it since I got it back. |
| What do you have for a scope? I was having similar issues with a .223 Tikka and was getting frustrated with the rifle. I had a decent quality scope on the rifle with an adjustable objective. I replaced that scope with a Leupold 6-18x AR scope and the rifle shoots much better now. Might have been some kind of parallax issue, my mounts were all tight. |
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Did you check the rings for alignment & lap the ring bore for the scope for any irregularities ?
I would look at a chronograph like was said . Try some factory match ammo to compare results . I use the lapping tool when bolting down rings to make sure they are aliened , of course one piece mount will not need that . Any stress on the scope will show up when the rifle warms up or cools ( shooting ). |
| Black Hills 168gr Match ammo is the most consistent. But consistency is what's throwing us. Sometimes the rifle hammers the target and opens up as it warms up through the 10 round test. Sometimes it scatters the pattern and then tightens up as it warms up. That's what has us scratching our heads. I've “heard” where DPMS recommends 200 rounds down the barrel before accuracy tests. I'm at 100 rounds, but have no idea how many they fired through this barrel. I only fired 30 rounds that day and the last test was the best. Not sure if the gun likes the hotter loads or maybe, it needs to run slightly dirty for the best groups. |
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I had an LR-308 with a 24 inch bbl. It would put three rounds through a single hole, then as the barrel heated the fourth would indicate a drift, all the rest in that string would go into a group of about an inch, but three to three and a half inches from my first three rounds. I would let the rifle cool down and try again. The first two would touch, the third would begin a new group some distance away. It was maddening. I sold the rifle.
I have wanted to get another, hoping that I had a lemon. I haven't yet though. PR |
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More round count may help ,but ya never know . I assume its got a free float hand guard .
I'm not a fan of there factory barrels , even though most shoot well. There are a lot of better ones available out there. Are you sure its at the proper torque spec's for the bbl. nut ? |
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Quoted:
Not for sure. I'd assume after a trip back to the factory and a return with what looks like a different barrel, it would have been torqued correctly. If I have to buy a barrel wrench, it will be to replace it. Do you have any way of changing out the mount/optics just to test with something else? Might be a good test to do before you change the barrel. |
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Quoted:
I've had mount problems, scope problems and mounts that shot loose on other guns. Not one exhibit what this gun is doing. Adjust the scope on this gun and you're repeating the same problem on a different area on the target. I'm just trying to help you nail down the problem by ruling out possibilities. Yes, it may move the problem to another part of the target, but that may still be a problem with the scope or mount; that's why I'm saying try another scope/mount set up. I've had scopes that had wandering zeros. They are maddening to deal with, and could be exactly what you are dealing with, unless you've had this scope/mount on another rifle and verified it's consistancy to hold zero. |
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In my experience building AR's, when the gun would pound a bunch of shots in one hole and then throw one or two out, the twist was too fast for the bullet I was shooting.
I know I am the black sheep here and don't drink the arfcom fast-twist koolaid, but that was my experience. When I leaned out the twist, the problem would go away. Tony Rumore Tromix Corp |
| That's different. This gun has a 1/10 twist. My Remmy 700, which shoots pretty much everything I put through it fairly consistently, is a 1/12 gun. I do have some 175gr BTHPs from Black Hills that I forgot to take with me. I'll try those. If the gun straightens up with them, I'll know for sure I need heavier bullets. And I “heard” these guns like the heavier bullets. |
| My LR308B has been shooting the exact same way since I purchased it this past October. I've got probably 250+ rounds through her, and I would say out of 6 shots, 4 will print in a tight enough circle, but I will always have 1-2 fliers.. I've tried Prvi Partizan 168gr match, 150-180gr Winchester, and it shoots pretty much identically with all types of ammo. I'd like to get it on a lead sled with a new trigger to take out all variables, but it's starting to become a little frustrating. Scope is a Nikon Pro-staff 3-9x. |
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Quoted:
Quoted:
Not for sure. I'd assume after a trip back to the factory and a return with what looks like a different barrel, it would have been torqued correctly. If I have to buy a barrel wrench, it will be to replace it. Do you have any way of changing out the mount/optics just to test with something else? Might be a good test to do before you change the barrel. That is a idea I will switch up my scopes. Give me a week or two/ |
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I wonder if Mr Rumore might be on to something. Kinda like how some 1/7 ARs shoot light bullets fine and some won't shoot them for crap. I've never shot bullets heavier than 168gr through my Remmy that has the 1/12 twist. Does anyone have a twist rate chart for the 308s like I see with 223s?
Edit: I just saw one. 1/12 twist is good to 170 grains. 1/10 is good to 220 grains. |
| Very frustrating problem you have. Have you decoppered your barrel lately ? If your scope mounts are tight and you use quality commercial ammo then you are down to scope or rifle issue. If you think it is the rifle after you change out scopes and retest then send it back. |
| Gun was thoroughly clean all times out. Got better as the powder loads got hotter. I first thought it just liked the hotter loads. Then got to wondering if maybe the barrel liked to run dirty. That's the next test. Start testing with the dirty barrel. But the twist rate intrigues me. Armalite uses a 11:25 twist. Just a tad bit slower and mentions the twist is for bullets up to 175 grain. It's like my 1/12 twist AR-15 shoots light bullets better then my 1/9 AR. And the 1/9 AR shoots the heavier bullets better. It wasn't until Mr Rumore mentioned twist rates that fact popped up in my mind. |
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I've been dealing with these DPMS LR308's for a while now. Mine was very troublesome when new. It would be accurate as possible if I placed a single round in the chamber, by hand. No magazine.
So, my question is,,,,,,, have you tried this? Also try holding the magazine in your hand and feeding the rounds out. What I discovered is that the rounds were being damaged by the feeding. The brass would have 2 dents on the taper, 1 big, one small. They dents would disappear upon firing. But, removal of a live round was very telling. The problem was mostly with the LH stack of rounds in the magazine. It took some fairly careful re-work on my part to get the gun to function properly. However, it's now settled into a reliable and accurate gun. My friends all went out and purchased ones like mine. They all had similar problems. Part of the problem is that the rounds were getting hung up on the aft rib of the magazine, causing the round to tilt upwards slightly as it feeds. Maybe firing 5900 rounds through the gun would break it in, but I chose to tweak it until it fed properly and did not dent the brass. One friend now uses Magpul magazines and swears by them. I've not tried them. PM me for specifics. |
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