Issue with sighting in new rifle

[hadmanysons]

So I took my new rifle to the range yesterday to get it all sighted in and I couldn't I go low enough on the scope to get it centered. For some reason the elevation was bottomed out and my POI was 6" above center @ 100 yards.

Some info about the rifle:
Konus M30 8.5-32x Scope
EGW Picatinny base 20 MOA Cant
Remington 700 SPS Varmint .308
Burris XTR Medium 30mm rings
$40 Cheapo 6"-9" Bipod from local gun store

So then I realized my first mistake. In my haste to get the scope mounted before I went to work on Friday, I had forgotten to tighten the rings down to the base :banghead: But that turned out to be a good thing, because I had the scope and rings all the way forward on the base, so I decided to move it all back to the rear edge of the base and low and behold my POI was about 4" below center.

My question is this, why did moving my scope back and forth over the receiver/action like that affect my POI so much? I'm glad I was just an idiot and there was nothing wrong with the rifle/parts but I can't seem to wrap my head around why the POI would move?

Also, I have Medium rings but could easily fit the Lows on there, would putting low rings on there give me more upward travel or would that not affect it at all?

Here is a picture of the gun in it's Pre-Range configuration

 

[Maverick223]

Your scope only has 50MOA advertised elevation, which is 25MOA up and 25MOA down. Your mount is 20MOA down, so you only have 5MOA of elevation to bring the scope center up to the POI. That could very well be the root of your problem, especially if Konus rounded up on the elevation measurement.

 

[JDGray]

Moving the scope up the rail has no efect on POI, since your still looking down the same plane. Tightning the rings probably made the difference in impacting lower.

 

[hadmanysons]

Your scope only has 50MOA advertised elevation, which is 25MOA up and 25MOA down. Your mount is 20MOA down, so you only have 5MOA of elevation to bring the scope center up to the POI. That could very well be the root of your problem, especially if Konus rounded up on the elevation measurement.

Which is what I thought at first, but others who have the same rifle & scope combo have not had this problem.

Moving the scope up the rail has no efect on POI, since your still looking down the same plane. Tightning the rings probably made the difference in impacting lower.

This is also what I thought but when I would move it back and forth, which i did a couple of times, the POI did move up and down. And I did make sure they were tight each time I did it.

 

[MCMXI]

Moving the scope up the rail has no efect on POI, since your still looking down the same plane.

That would be true if the base had zero cant but he has a 20 MOA base. Any movement fore/aft of the scope will affect the elevation of the reticle relative to the bore and consequently will effect the POI.

Also, I have Medium rings but could easily fit the Lows on there, would putting low rings on there give me more upward travel or would that not affect it at all?

Lower rings will move the reticle DOWN closer to the bore which will raise the POI. Your experience with your scope provides a good illustration of the effect of moving the reticle up or down. You were +6" on the target with the scope all the way forward (reticle closer to the bore). You were -4" with the scope all the way back (reticle further away from the bore). So if you use lower rings, it'll be equivalent to moving the scope forward again i.e. raising the point of impact. A 20 MOA base lowers the reticle relative to the bore compared to a zero MOA base. For long range shooting, you need more movement of the reticle DOWN inside the scope. The 20 MOA base that you have combined with your 25 MOA of +/- reticle movement will give you a theoretical 45 MOA of movement of the reticle DOWN.

 

[MCMXI]

I'd definitley use low rings if that were my rifle. That'll raise your POI by a significant amount.

 

[hadmanysons]

That would be true if the base had zero cant but he has a 20 MOA base. Any movement fore/aft of the scope will affect the elevation of the reticle relative to the bore and consequently will affect the POI.

This is what I suspected after my trip to the range.

I also just measured the scope travel. From elevator stop from top to stop from the bottom I got 57 MOA total travel.

So assuming that my scope stayed where it was in it "before-range" position, if I got low rings that move my POI down to the center of the target? That doesn't make sense to me. It seems to me that it would move the POI up.

 

[MCMXI]

So assuming that my scope stayed where it was in it "before-range" position, if I got low rings that move my POI down to the center of the target? That doesn't make sense to me. It seems to me that it would move the POI up.

No ... lower rings would move your POI up as I stated in my previous post assuming that nothing else changes.

 

[hadmanysons]

Yeah, that sounds exactly like what I DON'T want to do. The POI was too high to begin with. I want it to move down.

 

[JDGray]

That would be true if the base had zero cant but he has a 20 MOA base. Any movement fore/aft of the scope will affect the elevation of the reticle relative to the bore and consequently will effect the POI.

Cant see how it could make 10" difference

 

[MCMXI]

Cant see how it could make 10" difference

It probably didn't, but moving a scope 2" back or forwards on a 20 MOA base without adjusting the reticle is equivalent to adjusting the reticle about 7 MOA (up or down) so it does affect POA/POI. In this case the difference between loose and tight rings had a significant (greater) effect too, but the base cant does come into play.

 

[hadmanysons]

I'll probably just stick with the rings I got for now and then see how moving it back affected my POI (and tightening the damn thing too!)

 

[JDGray]

It probably didn't, but moving a scope 2" back or forwards on a 20 MOA base without adjusting the reticle is equivalent to adjusting the reticle about 7 MOA (up or down) so it does affect POA/POI. In this case the difference between loose and tight rings had a significant (greater) effect too, but the base cant does come into play.

Always something to learn

 

[Maverick223]

I would look into better and lower rings, those appear to be the cheapo rings that came with the scope. They may be fine, but rings are fairly cheap, and I would need to go lower for a good cheek weld, so I imagine that you could benefit also. If you really have 57MOA the alignment of the rings could be your problem. It could be a bbl that tends to throw bullets up, but you don't need to change that. The next thing to try is to shim the base or rings (under the ring, NOT in it), to correct for the unnecessary taper. More than likely the scope is the culprit, it may very well have more "up" in the elevation than down, either by design or accident. If all else fails, replace the optics with something a bit better.

 

[hadmanysons]

I would look into better and lower rings, those appear to be the cheapo rings that came with the scope. They may be fine, but rings are fairly cheap, and I would need to go lower for a good cheek weld, so I imagine that you could benefit also.

As far as i've heard the Burris XTR's are not cheapo rings and they did not come with the scope. They may not be Leupold or Nightforce but they are pretty good for the money. And if I did go with lower rings, that would just exacerbate the problem.

We'll see what happens when I take it to the range again. If I find myself with lots of extra room to work with now I might get lower rings. We'll see.

 

[Maverick223]

As far as i've heard the Burris XTR's are not cheapo rings and they did not come with the scope. They may not be Leupold or Nightforce but they are pretty good for the money. And if I did go with lower rings, that would just exacerbate the problem.

Right you are, I was going off of looks, should have re-read the OP. I would still check the rings alignment. Whilst it is true that the problem may worsen with lower rings, it will worsen by fractions of a MOA, and is therefore a negligible difference IMO. If you get a good check weld, go with what you already have.

 

[MCMXI]

Whilst it is true that the problem may worsen with lower rings, it will worsen by fractions of a MOA, and is therefore a negligible difference IMO.

Mav, a 20 MOA (1/3°) base only differs by +0.035" in elevation from front to rear over six inches. Changing from a 20 MOA base to a zero cant base (everything else remaining the same) will change the POI by 20 MOA right? That's 20 MOA with something on the order of 0.018" difference in reticle elevation. Now imagine what a 0.25" elevation change would do if you change the rings. I don't think it'd be negligible.

 

[Maverick223]

Mav, a 20 MOA (1/3°) base only differs by +0.035" in elevation from front to rear over six inches. Changing from a 20 MOA base to a zero cant base (everything else remaining the same) will change the POI by 20 MOA right? That's 20 MOA with something on the order of 0.018" difference in reticle elevation. Now imagine what a 0.25" elevation change would do if you change the rings. I don't think it'd be negligible.

Right...but, the rings are not tapered (or shouldn't be), so if it lowers the scope by 0.25in., the shot will be 0.25in. high at 100yds, or 200yds, or 1000yds. The base changing taper by the same 0.25in. will have a dramatic effect on the POI and become greater with increasing range. It will change by a measure of angle, rather than the rings which change the measurement by a small, fixed amount regardless of distance.

 

[Dr T]

I had a similar problem on a Ruger #1. The alignment on the scope rail was off enough that I could not get a Leopold VXI sighted in.

I solved the problem with Burris Signature rings and Pos-Align inserts (see http://www.opticsplanet.net/burris-signature-pos-align-offset-insert-kits-individual-inserts.html for example). The Signature rings have a plastic gripper insert that won't mar the scope. The Pos-Align inserts are eccentric and you use them in place of the standard. After centering the scope reticle with the scope adjustments, you place it in the rings and rotate the front and rear inserts separately until you are reasonably close enough to the bore sight. Then use the scope adjustments for final sighting in.

And, yes, I should have sent the Ruger back to the factory to get it fixed. It was easier just to get the Signature rings for the Ruger.

 

[benzy2]

That would be true if the base had zero cant but he has a 20 MOA base. Any movement fore/aft of the scope will affect the elevation of the reticle relative to the bore and consequently will effect the POI.

Simply not true. 20 MOA is 20 MOA at the front and at the rear of the base. MOA is an angular measurement. The angle is set once the base is mounted. As long as the base does not move, your angle compared to the bore is the same regardless of where on the base the scope is mounted. Ring height as well will only change POI by the change in height of the rings. You are not gaining or losing angle with different ring heights. If you were giving a 0.25" change from one ring to the other you would see a huge change in POI, but that isn't the case. Both rings are equal heights and you are still only given your 20 MOA that the base gives.

So no, moving the scope rearward in the mounts is not going to change POI, assuming all your parts are machined properly. If a part is warped it may show a change in POI but if everything is to spec it won't shift.

 

[Dr T]

The eccentric ring solution I described will only work in the case of a warped or mis-aligned part.

 

[benzy2]

I wasn't quoting you Dr T. I do see how it looks that way. Yes, misaligned parts are a pain that often require a fix like the Burris rings. I have a Marlin 917v that has the base mount holes drilled off center. Fun Fun.

 

[Bobarino]

http://www.opticsplanet.net/egw-remington-700-picatinny-rail-tactical-scope-mount.html

problem solved. they make 10 and 15 MOA bases.

even with the 10 MOA base and 175 gr rounds, you'll still have enough adjustment at 1,000 yards.

 

[Maverick223]

even with the 10 MOA base and 175 gr rounds, you'll still have enough adjustment at 1,000 yards.

Are you sure? That seems awful generous for a mere 10MOA; I guess my 92MOA of up (40MOA base with 52MOA of up in the optics) was a little excessive.

 

[MCMXI]

benzy2, you're absolutely right. I realized that I'd got it backwards after reading Mav's post (#18) yesterday. Apologies to JDGray and hadmanysons for confusing the issue. I was thinking of the scope base being canted down whereas it's the barrel that's canted up ... comparing a 0 MOA base to a 20 MOA base that is ... since the top mounting surfaces of both bases are in the same horizontal plane when the rifle is shouldered. A stupid error I know.

So the important thing here is that loose rings or bases can wreak havoc on POA/POI.