Scope, MOA and mount question

[slipknot]

I am trying to understand the whole zero MOA or 20 MOA riflescope mount choice. I have a new rifle in 6.5 creedmoor that comes with a zero MOA picatinny rail mount and a Nightforce precision benchrest 12-42x56 scope with NP-R2 reticle. According to the literature the scope has 40 MOA of adjustment vertical or horizontal. I guess that leaves only 20 up or 20 down. I have a few shots thru the gun and have already adjusted the scope up a few times for a total of 7-3/4 MOA and it is still shooting about 7 inches low at 200 yards. With only 12-1/4 MOA of adjustment left to go and not wanting to be in the last 10% of unreliable adjustment area if I want to shoot out further to 600 yards or more. I read that for a scope with 40 moa of adjustment that a zero rail is a good choice, anything with 50 or more of adjustment then a 20 moa rail is good. But I am not even close to zero at 100 yards and it seems to me I should be near zero at say 300 yards then be able to use the turrets to adjust my shots form there. So maybe I need a 20 MOA rail. ??? Any help is appreciated.

I hope I explained my thought process clear enough.
thanks

 

[vicorjh]

Probably, I have 20 MOA base on my 6.5 with zero at 200. This has enough down adjustment while being able to dial up to 1K accurately. Scope I have claims 80 MOA adjustment. This might be helpful, from Ballistics question? 20 MOA base vs. 0 MOA base

"The main purpose behind a canted base is to permit elevation adjustment of your optic at extended ranges. With a 0 MOA base, your optic is roughly aligned (should be depending on the profile of your receiver) with the bore centerline. If your optic has 50 MOA total of elevation travel, then with a 0 MOA base, the elevation adjustment is probably close to center with a 100 yard zero, leaving you 25 MOA adjustment up and down.

Long range hunters would rather use that 25 MOA worth of adjustment towards long range elevation travel...so they mount 5, 10, 15, 20 etc MOA bases to their rifles to extend the useful elevation range of their scope.

In the example above, and if the optic was centered with the bore centerline on a 0 MOA base.....if the 50 MOA optic was mounted on a 10 MOA base, the optic would have 35 MOA travel "up" (bullet path movement), and 15 MOA "down".

In the same example, if mounted on a 20 MOA base, it would now have 45 MOA travel "up", and 5 MOA "down". This might be considered marginal on the "down" adjustment remaining, to ensure the capability to zero at closer (100-200 yd) ranges.

Another factor to consider, especially with factory actions, is the profile of the action can contain significant alignment errors (both windage and elevation, but will focus on elevation for this post), that can "rob" your optic of some travel (sometimes as much as 5-7 MOA).

Additionally, some optics, have been known to "walk" or introduce a windage error at the extreme limits (top or bottom) of their elevation travel range...it's thought best to try a place the elevation adjustment towards the center of it's operating range, to minimize the chance/effect of this, no matter the quality of the optic."

I'm being kind-of lazy to put this to pen and paper. If you have a ballistics app and a rough idea of your BC, you can get an idea of how much adjustment you'll need for that extended range. Then determine your shortest range and find a happy medium.

 

[vicorjh]

Example with sight height over bore 2.75in, Hornady ELD-M *advertised* BC, MV=2733fps: 1000 yards = Up 27.3MOA, 600 yards = Up 11MOA, 400 yards = Up 4.8MOA, 200 yards = zero, 100 yards = Down 1.2MOA, 50 yards = Up 0.3MOA.

Notice how little adjustment "down" one needs for closer range than zero.

In your case, you might be pushing it at 600 yards assuming similar ballistics. Your actual ballistics and MV will be the determining factor for 600 yards with a zero base.

 

[slipknot]

Exactly, that is why I think I need to change to a 20 from the zero that came with the rifle. It is a Tikka T3x CTR 24” stainless

Nice rifle but I was surprised how far I having to bring the scope up to get to zero. Left to right is spot on centered for now.

Thank you

 

[mac1911]

Remember your scope is the front sight.
So your reticle is pointing lower, you can gain a few now by mounting the scope as low as functionally possible.
This might be over kill but I only know one person who shoots 600 yards and out.
He uses Burris XTR signature rings and basically used those to zero his scope at 200 yards with the moa rings on the signature series scope rings. He has a 20moa mount. On there also which gives him plenty of adjustment left.
IIRC he adjusted his elevation to impact 10moa high of point of impact then installed the ring inserts giving him 10moa above and 20 moa below that prime center adjustment range ?

 

[vicorjh]

Exactly, that is why I think I need to change to a 20 from the zero that came with the rifle. It is a Tikka T3x CTR 24” stainless

Nice rifle but I was surprised how far I having to bring the scope up to get to zero. Left to right is spot on centered for now.

Yea, I agree, I'm thinking you are correct and will be happier with a canted base. 6.5 is made to reach out there. On the other-hand, unless you have a 500+ yard range available or are planning to engage in PRS or something, you might consider leaving it alone until the need arises.

As mac1911 mentions, consider getting rings that will keep your optics low (if not already). Not only to gain some elevation but also because any left/right cant will create a significant error at range (an error that increases as the scope above bore increases). If you are planning on using this in other than benchrest situations, avoiding cant becomes a challenge. Whatever is comfortable is the most important thing, though. And, for that matter, replacing good rings is $$$.

Congrats on your new rifle. Looks nice. What's the approximate weight with it decked out?

 

[northny]

Go ahead and add the 20 moa base. I did it last year with .308 Win shooting hornady 168 grain match bthp . Caused no problems with obtaining zero at 100. Theoretically I have more than enough adjustment to go out past 1000 yards. In practice I had no problem with elevation (30” and a bit) shooting out to 924 yards. Just never tried going past 924 yards.

Then did same with 243 Win. Again no problem with close zero.

 

[slipknot]

Thank you,
I put Nightforce ultralight rings on it and it is low to the barrel now, I can barely get a velcro strap between it for my gun case. The rings are excellent. I splurged and got the top ring with a bubble so I can watch the cant.
Not sure what it weighs but the rifle is pretty light, the scope not so much. I like benchrest shooting and I will run it like this for a while and procrastinate on the swap while I get used to the gun. I read that the factory mounts are a bitch to get off, they have a tenon stud(which is great with the recoil)and I think it is epoxied, threads are blue locktite. It just makes sense to change it out to a 20. I'll try to get up a picture when I can.