Question about scope height above center bore

[LapuaFreak]

I have an AR-30 in .338 Lapua (10min sight base) with a Millett LRS-1 scope using the factory rings (these are very high rings my scope was 2.7" above center bore) that came with the scope. Believe it or not, that $500.00 scope has gotten me out to 1800 yards more than once, but, that's about it.

The scope touts 140 MOA of elevation adjustment. For my rifle to be Zero'd at 100yds with the factory rings, I had to dial up 4 full rotations, each rotation contains 15MOA, so that's right at 60MOA of elevation dialed up just to get me Zero'd. Seemed a bit much, so I did some reading and 'thought' I learned that the higher your rings hold your scope above center bore, the more elevation in your scope you eat/sacrifice to get yourself Zero'd. (do I have that right?).

Point?

I just got and installed a set of Low rings from Millett. Dropped the height of the scope a full 1/2 inch closer to center bore. Since I lost my zero by removing the scope & rings and 'thought' I'd be dialing down in elevation anyway (from where I was), so I just said hell with it and dialed my elevation touret all the way down and bottomed it out and figured I'd just dial up to 1 full rotation to get a Zero, rather than 4 full rotations (and eat up 60 MOA).

The outcome is that when I went to the range today to re-zero the rifle, turns out I ended up dialing all the way back up to that 4th full rotation of the elevation touret to get my zero, so it would seem that I'm still eating all that MOA, so of the 140 the scope has, I'm using 60, just to get a Zero, so that only leaves me with 80MOA (or thereabouts), so I have the same problem of maxing out that I did before I got the lower rings. Scope now sits 2.2" above center bore rather than 2.7" with the other rings.

Am I missing something? Is my understanding of how that works just bass ackwards?

Sorry for the long read. Any info you'd be willing to provide to help me understand would be greatly appreciated.
Thanks for reading if you did and thanks in advance for responding.

 

[Centurian22]

Welcome lapuafreak!

I'm interested in the answers to this but unfortunately can't provide much other than some youtube references. 8541tactical and tiborasaurusrex. From the little research I've started to do into med-long range shooting I would think the angle between scope and bore would have much more effect than the parallel height.

Take this example. If your rifle was locked down in a vice and you shot at X distance, then you raised your rifle exactly 1/2" (keeping it perfectly parallel to the original position) and shot again, theoretically (in a perfect world) your impact would now be 1/2" higher. No matter what X distance is 100, 300 or 1000 yards it shouldn't matter.

You are in effect doing the same thing with your rings. Everything else stays the same and stays lined up you just shifted down 1/2" therefore still have 60moa to get to your zero possibly 59.5moa to be technical but unless you have an insanely precise system I can't see that being actually noticeable.

Same viced rifle example but this time you raise the front of the gun only by the 1/2" on your second shot and you're going to have a much higher POI. Also I think (stretching my brain on this one) the height difference would be greater the further the target distance.

SO, to gain more elevation you would need to look into a 20-40moa base to improve your total amount of available elevation in your scope. Now all that aside having your scope closer to the bore is better (assuming you still get a good consistent comfortable cheek weld) as it will 1) make less difference between your Point of Aim and Point of Impact at close range and 2) decrease the angle at which your line of sight intersects your ballistic curve (at least at close ranges) which will make your "zero" technically accurate for a slightly broader range of distances (90-110yards as opposed to 95-105yards. This is example only not specific). Not sure if it will make any practical difference for you but like I said before all things the same in terms of consistent and comfortable cheek weld, closer to bore = better.

Hope this helps and hope someone can come along with more experience than I to provide better info.

Again welcome!

 

[Grizzdude]

wow 60 moa to get to zero seems like a lot. First verify that your 10 MOA scope base is not installed backwards. I have a 30 moa scope base and my scope zeros with only 5 mils (around 15 moa) from the bottom. Make sure your mounts are sitting nice and flush too. If everything looks good you will just have to get a steeper cant scope base for that particular scope. But make sure that base isn't backwards, 60 moa just sounds like a lot to me.

 

[Centurian22]

The Math makes sense to me 140MOA total in the scope, divide by 2 = 70moa each side of center, mounted on a 10moa base 70-10=60moa to come up for zero (80 left for elevation). Not trying to be rude I just want to make sure I understand this all correctly myself. As I stated I'm trying to start getting into medium to long range shooting myself and always do a lot of research (its kind of an obsession).

Also I think this was one of the better videos on MOA bases if its the one I think it is.

http://www.youtube.com/watch?v=Ppuzt3xv4Go

 

[Haxby]

You have 140 moa of adjustment. 70 up, 70 down from center. And that's pretty much right where you are. Ring height has almost nothing to do with it.

As far as the effect of ring height on trajectory/line of sight, there are several on-line ballistic calculators that let you run whatever numbers you want to see how they work.
The statement that a lower line of sight will "...make your "zero" technically accurate for a slightly broader range of distances..." is not correct.
As far as a lower sight height being 'better,' you would have to define 'better.'

 

[BCRider]

So from what I'm reading if you want to keep lots of the adjustment range in reserve for long distances then you really want a 30 or 40 MOA base so the scope points upwards quite a lot.

That sound about right? Do they even make 30 to 40 MOA bases?

 

[JDGray]

^^^^^^Scope tilts downwards you mean^^^^^^

 

[LapuaFreak]

Thanks for the responses.

I think I now understand that it's the angle of the back end of the sight base that will give or take MOA from my scope rather than the height of the rings. Seems now like the only benefit I got out of changing rings was ergonomics, in that it brought the scope closer and line of site is better w/out having to 'turkey necking' to get it.

This may sound like a newb comment, but not being a scope aficionado, I don't know.
If a scope says it's got 100 or 110 or 140 MOA of elev adjustment, why in the world would that constitute 70 up and 70 down? I'm thinking to myself.."70 down? How the hell do you dial 'down' from 0"...meaning what the heck would I have a need for dialing down for? What would constitute needing 70MOA down? Not sure why a scope manufacturer would do that. Kind of doesn't make sense to me at the moment.

If i need to dial down from 0 (assuming it's from 100-300yds) how the heck would I need 1/2 of my scopes total elevation to do that? (most scopes seem that way too) I can't get any closer to myself than point blank right?

So lets say I'm zero'd at 300yds, need to hit something at 50yds, I obviously dial down for that and it would seem, that to take it down to point blank, would be to dial down to mechanical zero? Yes? No?

I guess I just don't get why we all apparently deal with scopes that tout 100 or 120 MOA of adjustment, but in actuality you're 'technically' getting about 1/2 of that because of this 50up and 50 down thing or in my case 70 up and 70 down.

I don't see the logic in 70 down. It would seem that to start with, there's no where to go but up. I'm reasonably intelligent, but this is confusing the crap out of me.

 

[LapuaFreak]

Keep in mind, it's about 60MOA from what I assume to be mechanical 0.
I dialed my elevation touret all the way down to bottom it out.
From there, 4 full rotations later (15MOA per full rotation) I'm at a 100yd zero with my rifle.

 

[Grizzdude]

Most scopes are not centered like that 70 down and 70 up, that is kind of point less to loose all the elevation like that. My NF scope sure doesn't center in the middle of the elevation adjustment.

 

[LapuaFreak]

I need to suck it up and spring for a real optic. I've asked folks over the years that were sporting S&B's, USO's and NF's at the range if I could have a look through their scopes and one thing that's undeniable is the clarity and quality of the glass in comparison to mine.

If the Millett LRS-1 I'm using, actually zero'd at 100yds using somewhere around a handful of minutes above mechanical zero (bottomed out), I could work at shooting to the limits of the caliber, but I can't get much past 1800yds with a 10min sight base and what's left of the elevation in my scope.

I don't know of anyone that makes a higher sight base for the AR-30 either. The gen1 I have is a good rifle for what it is, but I'll be damned if anyone (including Armalite) makes much of anything for it in the way of after market add-on's or upgrades.

Probably have to find someone to custom make one for me, unless someone here knows.

 

[Highcaliber]

You're shooting a rifle chambered in .338 Lapua Magnum. I'm thinking you might want to consider a 300 yard or even a 500 yard zero. A 40 moa canted base would help if you want a 100 yard zero.

Now, regarding the "total elevation". Your elevation adjustment should fall roughly in the center of your total elevation adjustment range when zeroed.

The following info might help you.
http://demigodllc.com/articles/practical-long-range-rifle-shooting-optics/?p=4

 

[taliv]

it makes sense for the scope manufacturer to list the total MOA of travel. it's just a screw and they're telling you how long it is. (well, it's a little more complicated in some...)

anyway, in theory, "mechanical center" of your scope if everything is machined properly, should put it parallel with your bore. from there, you have to move the lines to intersect at 100 yards, AND to counteract the affect of gravity on the bullet.

of course, they're almost never machined perfectly.

now, what many people do is use 20 MOA bases (or 30 or 40 MOA bases). That effectively changes the angle the scope is sitting at from parallel, to pointing down toward the muzzle by 1/3rd of a degree (20 MOA). That change effectively gives you 20 MOA more adjustment in your scope, since you don't have to move it as far from mechanical center to get to zero.

so if you have 140 min, and 70 min is mechanical zero, and you come down 10 from that, for actual zero, you have 80 min of 'up' in the scope. a 20 min base would mean you'd have 100 minutes of "up" available to you.

as for your 1800 yards, i'm not sure what you're saying. if your wind gusts are bothering you at 100 yards...

 

[MCMXI]

I have an AI AWM in .338 LM with a Premier Reticles 5-25x56mm scope in an AI 45 MOA mount. 45 MOA is about 13 mrad and the scope has a vertical adjustment of +/- 15 mrad (+/- 52 moa). The scope and mount combination give me about +28 mrad of vertical adjustment which gets me beyond 2,000 yards with a 100 yard zero shooting a 250gr Lapua Scenar at about 2,950 fps. I need +24.6 mrad at 2,000 yards. Of course, I have the option to dial and hold over using the Gen 2 XR reticle which would give me an additional 10 mrad or 38 mrad total.

 

[Centurian22]

Highcaliber, that is an Amazing source of info! I can't wait to work my way through the whole thing.

 

[Grizzdude]

Check out Ken Ferrell, he probably already has a rail for your gun and if not he can whip one up.