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Discussion Starter #1
I recently helped my brother-in-law zero his LR-308 with Nikon M-308 scope. Nikon's manual directed that zeroing be done at maximum magnification (in this case 16x). We started with the rifle set in a lead sled and got a mechanical zero on the scope and rifle at 100 yards. It grouped surprisingly well, even for being in a sled. Once the rifle was zeroed in on the sled, we moved it to a bench rest with sandbags under the fore end and stock. The rifle continued to group very well, but did so about 3.5-4 inches above the center of target. I fired six out of it and got the same result. I expected some adjustment being needed after moving from the sled, but 4" was far more drastic than I expected at 100 yards. I got him back to a zero from the sandbag, then we transitioned to the bipod. The rifle again grouped well, but was again almost 4" high. Myself and another friend shot six each through it and they grouped well at the same position, 4" high.

As a test, I checked the zero on my Remington 700 SPS Tactical (.308) with Nikon 4-12x scope. I fired from sandbag rest and grouped as expected. I deployed the bipod and fired from it. My rifle grouped well and in center.

I am new to scoped rifle shooting, but have fired AR series rifles for years, including marksmanship instruction in the Marine Corps. This issue has baffled me. The consistency of the groups from three different shooters is a virtual guarantee that the issue is not a shooter problem. At the same time, the rifle/scope zeroed great on the sled, indicating the hardware seems to run right. Because my SPS did not have the same issue leads me to believe there some problem specific to the LR-308 that I'm not thinking of.

I have considered an ammunition factor, but we fired several brands and weights with the same results. My SPS will not group under 4 or 5 inches at 100 yds with 147 grain loads. When I switched to Federal 168 grain ammo I immediately dropped my groups to half inch on a good string and rarely more than 1.5 inches. Has anyone experienced a certain ammo type deviate 3-4 inches in consistent groups? I've never seen an ammo issue group well like this.

If anyone has had the same issue with this model (or any other model), I would appreciate some input.
 

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The Hog Whisperer (Administrator)
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Bipods, sleds, and bags can all group differently. As can shooting from prone, standing, with or without a sling, etc.

You've just confirmed that.

Welcome...
 

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Hello Ecko and welcome to the forum.

Since you got good groupings despite the gap between rested positions I'll assume the scope is completely secure but that would be the first thing I check or if needed recheck.

Did you try shooting it offhand?

Do you have a chronograph to see if you have changing velocity for some reason?

The issue could be ammo, some times strange things do happen like you are experiencing. I have had ammo that would group in 3's then go somewhere else, and repeat that process.

Hopefully we get some more people to provide good info to help you get your rifle on track.
 

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Discussion Starter #4
The scope is tight. I am working on borrowing a chronograph and will see what that tells me. I broke the rifle down at the range visually inspected it. I saw no evidence of damage in gas system or bolt carrier group. The trigger is very creepy, but everything in the fire control group looked intact.

Thanks for the input. I hate not being able to explain why a rifle does something like this.
 

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It may be something as simple as the position of your stock and the scope picture and distance of the scope from your eye causing parallax error caused by the angle and height difference of each shooting aid.
 

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It could be a parallax issue. Also, when you switched from sled to bags, to bipod, at any time did you shoot with the barrel touching any part of the sled or bags? I know that rifle comes with the free float hand guard and as long as that was the only thing being rest on that should not affect accuracy but if you rested the barrel on something instead it would more than likely shoot high.
 

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Discussion Starter #8
Good thought. One of biggest aggravations is seeing any shooter rest his barrel against anything so definitely wasn't an issue. It looks like parallax may be the problem. I'm researching it more now, starting with the link posted earlier. Once I get the theory down well, I'll apply it at the range and let you know what happens.
 

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The Hog Whisperer (Administrator)
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I would really be surprised if parallax is the issue. You'd almost have to work at it, putting your head so far over to the edge of the field of view in the scope for one setup, then the opposite direction for the other.

It is a pretty well known phenomenon that resting your gun against a hard surface, the point of impact may jump. You have several different surfaces that the gun may recoil differently from. It also make make a difference with the bipod attachment, etc.

Repeat your test to confirm the earlier results (which will rule out scope/mounting problems). Not all bipods will let you shoot to the same point of impact as sandbags, holding the gun offhand, etc.
 

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Welcome to the Shooters Forum!

Ammo will make a difference in group size and point of impact, and the difference will.vary depending on the particular rifle.

My experience, shooting off bipods will change the zero, especially with ARs having 2 piece handguards.

Probably don't need to mention this but will anyway, when you were set up on the bags , could the rifle recoil smoothly without hanging up on things like accessory rails and sling mounts? Its something I have to watch when using bags.

Concerning the sled, I haven't used one but hear of others having issues of the zero changing when using them.
 

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Discussion Starter #11
It seemed to recoil normally when I fired so I'm assuming the same is true for the other 2 that fired. I'm just basing that on similarity of point of impact and group sizes.

The sled is a great tool, but with limited application. It allows to strap the rifle in tight and zero the scope to the rifle. The mechanical zero can show you if there is a problem with your hardware. After that you still have rezero the setup to you.

In the sled, the rifle can't recoil much, if at all, so there is a bit of readjustment needed when you take it off the sled.
 

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To get a idea of how the parallax is with that scope set up put the rifle in the sled and while looking through the scope move your eye and head up and down, side by side, back and forward while looking at the target at 100 yards which should tell you the amount of error that is possible at that distance. It works the same as the peep sights when you move your eye and head around changing the position of the post in the peep. Its a huge problem with the peep while trying to shoot with a gas mask on because of the contortions needed to align the sights.
 

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I think Monty is more in line with the probable problem. Does this LR 308 have a front hand guard that free floats the barrel? If not, you might want to consider changing it over to one if you are going to be shooting off different types of rest with it sitting on the front guard. I have a feeling it's a problem with the hand guard and barrel mounting.
 

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Is there plenty of clearance between the front edge of the hand guard and the gas block that DPMS uses? I know they use the railed gas block; and the free float tube style hand guards can be pretty close to those at times. If pressure was exerted on the hand guard and it was touching the gas block that could really goof some things up with grouping and shift in POI.
 

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Put a 10 round magazine in it and try it with the magazine resting on each of the shooting surfaces instead of the hand guard. If POI doesn't change then, it has to be something with how the hand guard is interacting with the barrel. I have two 223's, one AR-15 and one M-4 Carbine, both have the standard non floated hand guard but I've never cared about the pin point accuracy of these. I've also shot thousands of rounds through M-16's and M-4's in the military but even with that, I've never tried them with the hand guard sitting directly on a rest, so I don't know it that would be considered a normal interaction or not.
 

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Discussion Starter #18
I will give that a try just to see, but remember that we've been doing all this at 100 yards. In my experience with non-scoped rifles, most mechanical and environmental factor will not have this dramatic an effect on a .308 round. I'm leaning towards an optics issue because that is the part of the equation that I'm not sure of with the rifle.
 

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One other thing. Make sure the scope mount is tight and the rings on the scope are also. Then. use a drop of nail polish or super glue and put a drop on both sides of rings on top. Also try to get a drop some place on the mount at a point where you can see if it cracks. Doing this will let you immediately see if you are getting any movement in the mount and scope. I had a very similar problem last year with a rifle I had just changed scopes on. I could shoot three shoot group that was great, make a correction to move the group to center bull and the next group would not be anywhere near where the correction I made should have put it. Six, three shot groups, that were great but no one where it should have been. I was chasing that thing all over the target. After going back through everything, after over 50 years of doing this, and one of the few times I didn't put nail polish of super glue on everything, dumb**s me left the front ring snug, not torqued. I've seen heavy scopes move in the rings when you would swear they were already over torqued. I have even had to go as far as putting a drop of green locktite between the ring and scope before torqueing it. If you do this, be sure not to get it near the screws, or you may not get them back out without some heat.
 
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