Rifle Scopes Why does 20 MOA base not give 20 MOA Elevation?

[kyle]

I've bought a Nightforce 12-42X56 Benchrest scope for my new 6.5x47 Lapua F-Class gun. Scope spec'd for 40 MOA elevation range. I'll need about 29 MOA to get to to 1K with a Berger 130 from a 100Y zero. Gun still being built and will have 20 MOA tapered base installed.

Just for grins , I mounted/zero'd the new scope on my existing .308, which also has a 20 MOA base. Lapua 155s at 2920 fps zero'd at 100Y. When I checked elevation range, I found I had 10.5 MOA down and 27.75 MOA up. That's a total of 38.25 MOA which is pretty close to what I expected since I also had to dial in a little windage.

What has me concerned is that the limited up elevation. I thought a 20 MOA base would essentially bottom out the erector once the scope was zero'd. Possibly not even be able to zero at 100Y but that I would have essentially the entire 40 MOA available to dial up. But the 20 MOA base apparently only gave me about 9.5 MOA of downward adjustment. Called Nightforce and their only explanation is that the barrel may not true to receiver. But this is a GAP build so I have a lot of confidence that everything is true.

What am I missing? I'm sick with worry. Thanks for your help.

 

[Supersubes]

Re: Why does 20 MOA base not give 20 MOA deflectio

There are a number of things that could cause what you are describing such as a receiver that isn't true on top, barrel / receiver misalignment, base not on straight,or a bore that isn't true. I have a rock barreled gun with a bore like a corkscrew and when the bullet exits, it goes down slightly. Gun shoots great. Don't assume that your scope will mount perfectly to the gun in the middle of your elevation travel(minus 20 with the base).

 

[NaHa]

Re: Why does 20 MOA base not give 20 MOA deflection?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kyle</div><div class="ubbcode-body">

What has me concerned is that the limited up elevation. I thought a 20 MOA base would essentially bottom out the erector once the scope was zero'd. Possibly not even be able to zero at 100Y but that I would have essentially the entire 40 MOA available to dial up. But the 20 MOA base apparently only gave me about 9.5 MOA of downward adjustment.


</div></div>

With a flat mount, you will be several MOA, typically, above the midway point of a scope's mechanical elevation adjustment range.

For example, on a flat mount, a scope I have that features 56 MOA of total 'up' has 24 MOA of 'up' remaining from a 200y zero. Meaning, 4 MOA above the midway point.

Your setup uses a bit more 'up', likely because of the factors already mentioned.

Here's your dilemma. You've chosen a scope with a very limited adjustment range. To be on the safe side, you should allow yourself at least a couple of extra MOA at the very top in case your load velocity varies due to humidity, altitude (or lack thereof), temperature, whatever.

So, sounds like you might want a 25 MOA or 30 MOA taper, on that specific rifle, with that scope.

Don't worry, be happy!

 

[MAX100]

Re: Why does 20 MOA base not give 20 MOA deflectio

You could shim the back of the base. A shim that is .005" thick will give 5" @ 100 yards.

GC

 

[doc76251]

Re: Why does 20 MOA base not give 20 MOA deflectio

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MAX100</div><div class="ubbcode-body">You could shim the back of the base. A shim that is .005" thick will give 5" @ 100 yards.

GC </div></div>

Actualy IIRC a .010" shim on a SA will buy you ~7 MOA.

Cheers,

Doc

 

[rijndael]

Re: Why does 20 MOA base not give 20 MOA deflectio

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: doc76251</div><div class="ubbcode-body">Actualy IIRC a .010" shim on a SA will buy you ~7 MOA.</div></div>

That seems pretty reasonable.

http://www.twincityrodandgun.com/Sight%20Calc/SightCorrectionCalc.htm

 

[Ol Rob]

Re: Why does 20 MOA base not give 20 MOA deflectio

Make sure you tighten the fron screws of the mount down first. If there is any gap under the rear of the mount at this point it must be bedded. That could very well be your lost MOA. I typically see around 18.5 MOA gained with a 20MOA base.
YMMV=Rob

 

[craigp40]

Re: Why does 20 MOA base not give 20 MOA deflectio

I just went through this issue and this might help you:
Elevation Discrepancy

Just bedded my bases on a couple rifles last night, which should solve the problem I saw.

 

[gtofan]

Re: Why does 20 MOA base not give 20 MOA deflectio

I had that exact same dillema when I was having my 260 built. Started with a Monarch, then debated the 12-42 BR, ended up with the NXS solely for elevation.

Mine is roughly 11.5 MOA from top to get to 100 yds. Depending on the reticle you could use the reticle for 100 if needed.

 

[kyle]

Re: Why does 20 MOA base not give 20 MOA deflectio

Thanks for the comments. The 6.5x47 is being built on a Barnard action. I bought the 20 MOA base from Mac Tilton specific for the Barnard. So I don't believe a Badger or Nightforce will work but I need to check with Mac.

Seems like shimming is my best option. Once we get the new gun built, I'll see where it zeros. Then I can fine tune the shim thickness to put the erector 3 or 4 MOA off bottom. That should give me about 35 MOA up elevation. I could even shim it so that I zero at 200Y. This is dedicated F-Class rifle so I doubt I would shoot it at distances less than 300Y, ceretainly not less than 200Y.

Am I thinking straight?

 

[NaHa]

Re: Why does 20 MOA base not give 20 MOA deflectio

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kyle</div><div class="ubbcode-body">Thanks for the comments. The 6.5x47 is being built on a Barnard action. I bought the 20 MOA base from Mac Tilton specific for the Barnard. So I don't believe a Badger or Nightforce will work but I need to check with Mac.

Seems like shimming is my best option. Once we get the new gun built, I'll see where it zeros. Then I can fine tune the shim thickness to put the erector 3 or 4 MOA off bottom. That should give me about 35 MOA up elevation. I could even shim it so that I zero at 200Y. This is dedicated F-Class rifle so I doubt I would shoot it at distances less than 300Y, ceretainly not less than 200Y.

Am I thinking straight? </div></div>

No, you're not.

The base you bought from Mac is specific to the Barnard. Neither Nightforce nor Badger Ordnance make a base to fit the Barnard.

Forget the shimming. Wait until you get the completed rifle, then mount the scope, and zero the rifle at 100y. Then count the MOA that remains to the top of the adjustment range. If it's enough, with a couple of MOA to spare, then you're good. If it's not enough, then you can consider shimming, or depending on the reticle featured in your NF scope, you could use holdover aiming, i.e. if your reticle has intermediate points to use as a reference. With regular crosshairs and the like, this idea will not work.

 

[ker2222]

Re: Why does 20 MOA base not give 20 MOA deflectio

also make sure you have your rings spaced as far apart as possible. The 20 MOA counts from front to back of mount. If your rings are both towards the middle, you aren't going to get the full 20....only the fraction that your rings span. Probably about 10MOA..............ask me how I found this out

 

[sandwarrior]

Re: Why does 20 MOA base not give 20 MOA deflectio

ker2222,

That would certainly make a difference if you had two piece bases. It shouldn't make a difference with a one piece base. The body of the scope should be parallel to the top of the one piece.

Kyle,

You could use the same formula that you find MOA with at yardage to find out how much your scope is going to be affected. That is: 1/60 {=.016667} sin{=.000290888...} x 4 (inches of one piece base or center to center of rings) = .00116" So it takes .00116" elevation to make one MOA difference over four inches on the top of your rifle.

 

[ker2222]

Re: Why does 20 MOA base not give 20 MOA deflectio

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sandwarrior</div><div class="ubbcode-body">ker2222,

That would certainly make a difference if you had two piece bases. It shouldn't make a difference with a one piece base. The body of the scope should be parallel to the top of the one piece.

</div></div>

it will even make even more of a difference on a one piece base. If you think of a 20MOA rail as a slope (rise over run)...........the full rise of the slope is only achieved if your two points (rings) transverse the entire run of the base. If your two points are close together, you only get the fractional increase in rise equal to the fractional part of the run transversed.

I am not sure if I am explain it right. But, I experience the exact same frustrating experience described by the OP. Except mine seemded to defy physics to me for a while. I first mounted the scope with the rings towards the ends of the rail. The sighted in and checked what the available MOA increase actually was....worked out to like +19MOA and I was happy. I then moved the rings and scope around becasue I didn't like the eye relief. When I went back to the range, I had somehow lost about 9 MOA in travel. I was utterly stunned and confused and assumed I had somehow bent my scopetube or something. After a little sage advice.........I put the rings back towards the ends of the rail.....a little more towards the middle than orignally mounted but still much further toward the ends. When I went back to the range......I was up to about +17MOA which was enough......

 

[gugubica]

Re: Why does 20 MOA base not give 20 MOA deflectio

ker, you have some other issues going on. It should make absolutly no difference in angle how close or where you rings are mounted on the rail, unless:

your rail is bent (which is possible if you the surface it was mounted on was not true, it could have bent).

your rings are not the same height.

If your rail is true and you rings are the same height above said rail, the scope tube and the rail should (theororetically) be paralel. It would not matter what possition the scope rings are mounted on the rail if they are paralel.

 

[Surgeon_Shooter]

Re: Why does 20 MOA base not give 20 MOA deflectio

Fuck it get a 40 moa rail

 

[sobrbiker883]

Re: Why does 20 MOA base not give 20 MOA deflectio

Get the rifle being built in hand, mount the base and then see what you need to do.......

 

[Red_SC]

Re: Why does 20 MOA base not give 20 MOA deflectio

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gugubica</div><div class="ubbcode-body">ker, you have some other issues going on. It should make absolutly no difference in angle how close or where you rings are mounted on the rail </div></div>

Right, the scope tube should be parallel to the top of the rail, no matter where the rings are.

 

[TresMon]

Re: Why does 20 MOA base not give 20 MOA deflectio


Yeah, thats like the crazyest thing I ever heard: spacing of the rings does not change the slope.

That's like somebody saying two cars, a short little Mini-cooper and a super long wheelbase 4-door F350 truck both going up a mountain grade on the interstate, side by side, but the Mini Coopers climb is not as steep because it's wheel are closer together!

If you believe that, I got a Handy-Rifle in .338LM that is an 1/8 minute rifle I sell ya for $50.

 

[sandwarrior]

Re: Why does 20 MOA base not give 20 MOA deflectio

As said above...that's what I meant, theoretically. Scope rings of same height and true rail will make the scope center parallel to the rail. If trued, hopefully, also center of action/bore.

As noted the scope rail might not be true or tweaked because of differences in the mount heights of the rail vs. the points of mount on the rifle.

It is a lot more possible to have an inaccurate mounting from two piece bases. And what you're saying regarding moving the rings out as far as you can makes sense with two piece bases. That WILL decrease the average discrepancy of alignment. In my last post I used 4" center to center which comes out to about .001" to make one MOA change @ 100 yds. If you were to stretch that out to 6" difference it would make it closer to .002" to make an MOA difference.

 

[sobrbiker883]

Re: Why does 20 MOA base not give 20 MOA deflectio

Dude, get the gun built and then sweat the scope.......

 

[dgwelsh]

Re: Why does 20 MOA base not give 20 MOA deflectio

just a question,
what ker is saying might make a certain amount of sense in that the reticle, a single point, is moving along the base. At different points along the base you are going to get different heights which effects the POI and therefore the MOA adjustment. The spacing of the rings effects the MOA only so far as the where the reticle is placed. Is this correct? I may have had too much to drink.
dgwelsh

 

[hangunnr]

Re: Why does 20 MOA base not give 20 MOA deflectio

Nope, makes absolutely no difference as to where the reticle is in relation to the placing of the rings as far as MOA is concerned.

hangunnr