Best-practice for determining rail-elevation?

[Macintosh]

Or perhaps more accurately, "enough, but not too much". I think most folks know about using a rail with built-in elevation, i.e. 10moa rails and 20 moa rails, etc. The theory being that the center of the adjustment-range of any scope will be better optically and more likely to dial accurately and consistently, in addition to simply adding to the total useable elevation travel. My biggest scope has an advertised 32 mils of total travel. With my scope using a flat rail I only have 16 mils of total useable travel above zero, and the top of the travel (I have been told) is more prone to distorted optics, less accurate tracking, etc and should be avoided--since that is inside the range at which I've routinely shot targets, it would be limiting. A 20moa rail (5.8mils) is very-roughly 20% of the total travel in my scope, so it means between my 100-yard zero and about 20mils of elevation (roughly my 1400 yard solution with my 6.5mb) I am using only the center 60% of the scopes adjustment range, plus I still have another 5+mils to dial above this in a pinch. My questions are, am I thinking about this the right way, and what is the % of total travel or the absolute amount of travel at the top and bottom of the adjustment range that one "should" allow as a "buffer" like this (i.e. avoid using) by using an elevated rail? In other words, with any given scope, how do those in the know recommend whether you are best off with a 10moa, 15moa or 20moa rail AFTER you have achieved "enough travel" (or how do you know when you are in the "red zone" as a result of achieving enough travel?)?

There is tons of info out there on making sure you have enough elevation travel to hit a target at X yards using X scope and cartridge...but very little about what the specifics are of how far off the extreme end of the scope's travel one should stay. Below is the only sentence specifically quantifying this I've been able to find, and it really contains no context or reference so I have no idea how valid it is. Does anyone out there have good info or insight on this?

"...I like to stay at least 3 MIL or 10 MOA from the bottom for zero (the lower the quality of the scope the further I want to stay away)....." Quote from this link:

How to figure the correct MOA angled scope mount base....

These questions seem to come up a lot with the rise of people shooting .22LR out to 200, 300, 400+ yards.... Do I need a 0 MOA, 20, 30, or 40 MOA scope mount base ??? Can I still get a 25 or 50 yard zero??? How far will I be able to dial with XX scope and XX MOA base??? This depends on 4 main...

www.snipershide.com

 

[dieselchessy]

I’d say your musings are correct as to scope travel/rail pitch selection.
One other consideration is scope height, cheek weld, bell clearance, ect.. a 20 Moa rail will effect this noticeably.

In my line of work we tend to stick to the middle 80% of an adjustment range for mechanical type adjustments and mechanical type measurement equipment. So do not tend to use the bottom or top 10%.
Similar would probably carry over to scopes, being a spring loaded mechanical adjustment.
I haven’t tried to wrap my mind around the optical considerations of dialing to the edge of erector travel. Likely very dependent on scope quality, scope body length, ect…

I run 20moa rails. Anything less seems pointless and more messes with scope eyepiece height to much.
But if your shooting 1500+ yards then 30moa+ may be in order.

 

[Macintosh]

Thanks, thats helpful. In this case height is not an issue. Im asking not so much for extreme long range, but for “normalish long range”, but using a scope with less travel than a big 34mm scope might have. Wondering how far one can realistically push your zero down into the very bottom-end of the erector travel without causing problems, and having trouble finding answers that are more than “I’ve always been told…”.

 

[Macintosh]

It’ll depend on which specific optic you’re using and also what will be the majority use of the rifle.

IME, high quality scopes like ZCO don’t have the optical distortion or mechanical issues at the extreme edges. Cheap scopes, and some Rokslide favorites (like Bushnell Elites, SWFA) have shown progressive mechanical errors as you move towards the outer edges of travel. Every NX8 2.5-20 that I’ve messed with or shot has had noticeable optical distortion, and I would expect to see problems at the extreme ends of travel.

If a 20 MOA rail causes issues with your scopes performance, you should get a different scope. Without specifics, I’d say set-up your scope for 90% of your use.

Makes sense. I'm interested in simply understanding it better in general, but in this specific case it's a rifle I own that is sort of a very heavy deer rifle/very light PRS rifle. It's more or less my practice rifle, and I do all sorts of positional shooting with it as well as shoot the occasional local PRS match with it. It currently has a big 4.5-29x scope on it with 32 mils of travel, plenty for any shooting I ever do with some wiggle room on top of that, but I've found the scope impractical for most of my shooting outside of those occasional matches. This fall I used that rifle on a traveling hunt and felt I had to switch out the scope. I am planning to replace that scope with something that crosses into hunting better, while still allowing for the occasional 1100-1200 yard shot in a match. One of the scopes I've considered has only a bit over 17mils of travel--in that case a 15moa rail would mean the 1200 yard match shot would max-out the elevation and maybe even force a holdover, while a 20moa rail would have my zero less than 3 mils from bottoming out. I will obviously decide what to do based on what makes the most sense--certainly the 15moa rail seems more prudent and would work just fine for the majority of my shooting--I am just trying to figure out if the above scenario with a bigger rail is a problem at all, and if so how much of a problem. "I've always been told" it's a problem, just looking to understand better exactly where it goes from problem, to fine, to ideal...BEFORE I find out the hard way.

 

[dieselchessy]

You could use the Burris signature rings and a sloped rail to tune your mounting to what you need/want.

 

[Macintosh]

If its not possible to arrive at a decent rule of thumb due to variability then testing makes perfect sense. Just need to find a practical way to do that when I don't own the scope, no one I know owns it, and everything is online only and “not returnable once mounted”.

 

[Macintosh]

I guess you dont want to waste $ on ammo on a scope that doesnt work, I don't want to waste money on return shipping or a restock fee if I'm simply getting the wrong scope. For what it's worth, the manufacturer of the scope in question (Trijicon in this case) was adamant that they thought it was fine to have your scope zeroed right up to the point of resistance at the bottom of the scope travel without causing any problems--Not sure I'd plan to go that far, but at a minimum it's reassuring and would give me plenty of travel.