Mathematical Problem

[ramrod45]

Recently mounted a NXS 12-42X56 on my Remington 700 short action (.308 Win) using a 20 MOA base, so I can reach 1000 yds for F/TR competition . So the NXS has an "internal adjustment of 45 MOA" that's 22.5 up and 22.5 down from the factory center of the scope. In a perfect world, the rifle, base, and rings would all be perfectly made and installed. If so, I should be able to adjust the elevation on the scope all the way down to the stop and have 2.5 MOA up the ZERO. (I hope you followed me on that last part). BUT I DID NOT ! I still needed about another minute to a minute and a half down to zero the scope at 100 yds. So I tried to zero at 275 yds. (that was the longest range available in my area) NO GO. I still needed about a half MOA. So I said to hell with it. I shot at the bulls eye and hit 3 inches high for the ten shots. This is with the scope's elevation adjustment bottomed out.
So here's the mathematical problem. Since the shots are hitting three inches high at 275 yds, and the Hornady ballistic calculator is telling me to "come up in MOA" 31 minutes (the zero range input is 275) What should really be my UP adjustment ? Thank you to all you rocket scientist that are going to answer this question.

 

[pmarauder]

Let me guess..... its a TPS 20 MOA scope base??

 

[maxscm]

Sounds like your base is a 30 MOA.

and if Fletcher doesn't buy it,,,I'll double his offer. ;D

 

[skeetlee]

Since its here in the classifieds it must be for sale! Why else would someone post a question in the classified section? Lee

 

[Keith Glasscock]

Heck, I'll give you $500... For the scope - sell the base to someone else - sounds like it is out of spec.

 

[Aceball]

Have you checked your case lengths?

Sorry, I just had to do that.

OK, 3" high at 275 yards equals 1000/275*3 or 10.91 MOA already corrected at the 1000 yards mark. Therefore the requirement for a 1000 yard MOA of 31" is now offset by that +10.91" so crank up another ~20" (31-10.91). Should do the trick if you calcs are correct.

Maybe.

 

[george]

get a set of burris signture rings they come with + and - incerts put a -5 in the bottom rear ring you'll be good to go.

 

[jlmurphy]

My calculations show a typical .308 load needing 12 MOA to reach 275 from a 100 yard zero, 12 MOA/2.75=4.3 MOA at 100. I have the same scope, I used a 15MOA base made by my son at Murphy Precision. The tops of Rem 700's are not particularly true.

 

[Killshot]

My calculations show a typical .308 load needing 12 MOA to reach 275 from a 100 yard zero.

Uh, from a 100yd zero to 275 is only about 3moa....

Without using some Burris rings, OP has a choice, either have
a 100yd zero or shoot at 1.000 without holding over.
With only 45moa of elevation that scope won't do both.

 

[Nomad47]

The OP said he was 1 to 1.5" high at 100 yards. He also said he was 3" high at 275 yards which is 1.0909 MOA high. So if he took out 1 MOA (which he can't), he would be dead on at 275. And he would be 1/2" high at 100. Must be one helluva flat shooting rifle.

 

[Nomad47]

Ramrod, if your ballistics program has a "path" column, you could keep adjusting scope yardage zero until the "path" is +3" at 275 yards. Then you will have your true scope zero.

 

[Nomad47]

Well put, Dave Berg.

 

[BoydAllen]

Dave gave your answer. Let me add a couple of related details. Just because you are in the middle of your turrets adjustment range, with an equal amount of adjustment available in the up and down direction, does not mean that the reticle is centered within the OD of the scope body. To find that point of adjustment, you will need to use some sort of V block arrangement to adjust and turn until the intersection of the cross hairs does not move, relative to a target as the scope is rotated. Also, if you had to do much windage adjustment from center, that can change how much vertical adjustment is available. Although I have not tried it, or needed to, It would seem to me that a little judicious ring lapping , with more pressure on one than the other, might be useful to correct your problem(if you want to sight in for 100 yd.), since it takes so little scope movement to move POI an inch and a half at 100 yd. The same thing could also be done with bedding of the scope in the rings. I always lap my rings, and have, in the past bedded steel rings, given the amount of effort required to lap them, if they require very much to achieve full contact.

 

[Aceball]

I am not sure that ring lapping and bedding idea is so good at this point. (I do this for BR/Long Range scope mounting for a living at my gunshop, every day, heck even 2-3 times a day if I am lucky.) Best to leave things alone until you know exactly where the problem really is. To eliminate any scope issues remount a flat base and try the scope at 100 yards to see if its in reasonable range, and if so, replace that mount with a new 20MOA mount with Bench-Source brand or equal. My previous post was taking in account your imperical data provided. The other posts are also correct. Remember individual rifles produce different velocities so shooting paper at 100 and then at 300 is a very good method for determing true MOA estimations for the required corrections for those greater distances like 1000. Always reference back to 100 and work from there to avoid confusion.

 

[ramrod45]

I want to thank ACEBALL, JLMurphy, Nomad47, Dave Berg, and Boyd Allen for your excellent comments. I think it's going to be better if I have a custom base made so I well be able to properly zero the scope for what ever the caliber of bullet, and powder load. Which leads me to the next question. With the info from my original post, should the custom made base be a 23 or 25 MOA base ? Thanks again for your experience in answering this question.
As for fletcher, pmarauder, maxscm, skeetlee, and busdriver, you can all go play in the mine field.

 

[BoydAllen]

I have never seen a Remington on which the top of the rear receiver ring was at the right height and level. In order to do a proper one piece base mounting job, they should be epoxy bedded. Another thing that should be kept in mind is how small a difference we are discussing. I don't have a short action rifle out of the safe but if we use a generous center to center distance for the base mounting points 20 MOA (a third of a degree) is only about .026, and for the amount of difference that you are talking about, one fourth that, and amount that bedding the base could be easily be manipulated to produce. Just for fun, take the rings off the base and loosen all of its screws, and then tighten the front ones, and let us know what you see. Sometimes fellows treat this area as if it is a simple screwing together of parts. If you do it right, it most certainly is not, at least with typical factory actions.
Boyd

 

[Nomad47]

Ramrod, I'm confused. I thought you were saying the 20 MOA was too much. But now you're thinking 23 or 25 MOA. I use Burris Signature Rings with 20 MOA bases. With the two 10 MOA inserts that come with the 30mm rings, a lot of MOA combinations are possible. And if your windage is a little off center, that can be corrected as well.
I suggest you do as BoydAllen suggested but also go with the Burris rings.