Scope height questions

Discussion in 'Technical Questions & Information' started by jpdusn, Jul 7, 2012.

  1. jpdusn

    jpdusn New Member

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    Reposting here.

    First some info on rifle:
    Rifle: Remington R-25. (308)
    Mount attach point: Pic rail on top of AR style receiver.
    Questions:

    Is there a "standard" height above the pic rail or top of the receiver (1/4 inch difference) that the centerline of the scope should be?

    Do manufacturers of firearms or ammo recommend any distance specs for the distance between the centerline of barrel to the centerline of scope?

    Does this distance (greayer or smaller) effect accuracy?

    Is it just a comfort/ergo thing?

    Thanks in advance for any info on this subject. It will be appriciated.
  2. goofy

    goofy Well-Known Member

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    Hi jpdusn.
    What scope are planing to put on?
    There is no set height because depending on what optics you are going to use will determine the height.
    Style and size of the objective of the scope will determine it.
    And how you like to hold the gun (ware your eyes line up) should also be taken into account.
    There also is see thru mounts so your iron sights can be used.
    Remember you bore sight the scope after mounting then off to the range for final sighting it in.
    Mike
    Last edited: Jul 7, 2012
  3. Hammerslagger

    Hammerslagger New Member

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    Here is a link that is likely to be a bit more complicated than most persons are going to want to deal with. Still, it explains some basic principles. http://en.wikipedia.org/wiki/Rifleman's_rule

    Generally speaking, it is desirable to have the top of ones sights as close to the centerline of the bore as possible. Thus, the iron sights, on a conventional single shot or bolt action rifle, have some advantages over a scope, as the line of sight is closer to the centerline of the bore using iron sights.

    The higher the line of sight is above the bore, the further out (in horizontal distance) will be the crossing point of the line of sight and the bullets path. Thus, the higher the line of sight, above the bore axis, the lower a rifle will shoot at say 25 to 50 yards on say a 200 yard zero.

    Hope this helps.
  4. Bobitis

    Bobitis Guest

  5. BlackEagle

    BlackEagle Well-Known Member Supporting Member

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    jpdusn, welcome to the forum. Hope you can take some time to browse the other sections; it's a great place with great help from people who know their stuff.

    My experience falls in line with what goofy and Hammerslagger said. I have scopes on my rifles.
    The .223 has a large objective so sits relatively high off the barrel. The gallery rifle has a smaller objective so sits closer to the barrel. The scope on my .22 caliber AR15 look alike sits on top of the frame, a good 2.5-3 inches above the barrel. All three shoot fairly accruately for where they are sighted in, and then I make adjustments for shooting other distances.
  6. Jim K

    Jim K New Member

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    Unless the scope line of sight is very close to the bore line, or the target is at a very short range, the bullet will cross the line of sight twice, once on the way up and once on the way down, at the point where the rifle is "sighted in".

    As for accuracy in relation to scope height, there really is no connection. A scope could (in theory) be mounted ten feet above the bore line and the rifle could still be sighted in.

    So scope height, stock comb height, etc., are mainly matters of scope and rifle design, plus shooter comfort and usability, not accuracy.

    Jim
  7. JLA

    JLA Well-Known Member

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  8. LDBennett

    LDBennett Well-Known Member

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    The AR platform in its M16 configuration was designed for iron sights mounted on the top of the handle. The stock is so made to accommodate this. That is, the stocks upper surface where your face touches is very close to the center line of the barrel. That makes the sights have to be located pretty high in order for the iron sights to line up comfortably with the aiming eye. Any lower and weird head positions are required to even seen through the sights.

    With the flat top version of the AR, iron sights are almost unusable if comfort during shooting is desired. If a scope is mounted on a flat top version it must be raised quite a bit in order to align comfortably with the shooter's eye. The height above the barrel, almost regardless of the scope's bell diameter, is dictated by comfort of the shooter. Risers are commonly used under scopes on the AR flat tops to get the scope aligned with the eye of the shooter. It might be nice to have the scope down on the barrel for geometric considerations but with an AR flat top it is virtually impossible to have a comfortable shooting position with a scope down on the barrel.

    I have three AR style guns: a common 223 flat top varmint version, a 22LR Tac Sol flat top version, and a 50BMG equivalent BOHICA flat top bolt action conversion. All three require high risers in order to comfortably shoot them. In my case the riser might be extra high as I have a long neck but my son-in-law does not and he never complains about the scope mounting when he shoots our 50BMG (actually 50DTC) conversion.

    So the bottom line is really ergonomics. The scope needs to be high enough such that you can view the scope through the center of the scope's ocular (rear) lens system and not off center, which could introduce parallax errors in scope sighting.

    On guns other than AR's, the lower the better (usually), but you still have to have the scope high enough for comfort. A lot of scope height is dependent on the stock design. No matter what, the shooting position has to be comfortable and allow the eye to align with the optical center of the scope.

    Today a lot of people buy the big 50mm or larger belled scopes, forcing very high scope mounts. Those large belled scopes are probably a waste unless the scope is high magnification. These scopes are common in Europe because they allow low light shooting whereas, throughout most of America, shooting in other than daylight is not allowed. Any gains in brightness are lost by the limits of the iris of the human eye...but that is another story.

    LDBennett
  9. jpdusn

    jpdusn New Member

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    Thanks for the info guys. I guess I was pretty sure that the ergo/comfort/eye alignment is the over riding issue as that's what I was always told. Along with being told to always mount the scope as close to the top of the barrel as possible. This has worked just fine on all my hunting style rifles but does not work at all for me on this flat top R-25 and so spreading out the distance between the bore centerline and scope centerline for ergo/alignment only reasons was a concern.

    I believe a mount similar to the one listed above would be a great improvement. It would raise my current scope position by a half inch putting it in a more ergo position for me. What is a fair cost for a mount like this one? I'm kind of the middle of the road guy when it comes to purchases. I find problems with cheaper stuff and think the top end stuff is overpriced.
  10. Bobitis

    Bobitis Guest

  11. LDBennett

    LDBennett Well-Known Member

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    There are also less expensive risers that fit between the gun's rail and the scope mounts. I use them on my AR with no problems.

    LDBennett
  12. JPD

    JPD New Member

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    Well I picked up a new set of scope rings last night. I went with the Nikon P-Series 2 piece mounts. They raised the scope hieght almost a half inch from their previous location and they match up nicely with the Nikon 3-9 x 40 scope and most importantly, my eye. Much more comfortable and my head is in a much better shooting position. Now all I have to do is get to the range and site iin the scope......again. Oh darn, that really sucks.....NOT!!!

    http://www.opticsplanet.com/nikon-p-series-scope-mount-w-rings.html
  13. GunHugger

    GunHugger Well-Known Member

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    No, not always true.

    That depends on the zero distance, the caliber, the bullet, scope height and the rifle.

    In this chart an AR firing M193 ammo shows a close and far zero for a 50 and 25 yard zero, but when using a 100 yard zero the bullet only touches the line of sight at 100 yards.
    [​IMG]

    I like a 50 yard zero with my AR's since it will hit within 2" high or 2" low out to about 250 yards.

    I could discuss the second part of your reply too...

    In theory, you are correct. But it does matter. The higher the scope is mounted the more off it will be at any distance other that the one it is zeroed for. So the lower you can go is always best unless you shoot at the exact same distance every time.
    Last edited: Jul 11, 2012
  14. Jim K

    Jim K New Member

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    I agree in the situation where the line of sight is relatively hign (2 inches or more) above the bore line.

    And you are correct that "it will be at any distance other that the one it is zeroed for", but I didn't address the "other than" distance. I said that the distance of the scope from the rifle bore, up, down or sideways, doesn't prevent the scope from being sighted in, and that is true. Now a scope ten feet above bore line would not make much sense for a normal rifle, but artillery pieces often have their sights several feet from the gun bore, and remote controlled naval and coastal defense guns were sometimes "zeroed" from hundreds of yards or even miles away. If the projectile hits where the sights say it should, the gun is "zeroed" no matter where the gun and the sight are in relation to one another.

    Jim
  15. JPD

    JPD New Member

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    Couple of questions about your chart.
    1. The distance between 0 and the -2.5 on the left side of the chart. Is that the distance between the cross hairs/line of sight (0) thru the scope and the horizontal center line of the barrel bore (-2.5)?
    2. What exactly is M193 ammo?

    Other than that the chart makes sense. The bullet must always rise to meet the line of site at least once depending on site in distance and the trajectoy curve of your particular ammo and rifle. Did I get that right?
  16. JPD

    JPD New Member

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    So if I'm understanding this correctly I have a couple more questions for you guys:
    1. When ammo manufacturers (ie. Win, Hornady, Rem) post ballistics for their ammo, is that based on the "line of site" and "bore center line" being the same?
    2. How can scope manufacturers (ie. Nikon & others) calculate bullet drop for their rectiles (BDC) when they never ask for the distance between the scope center line and the bore center line or the barrel length for that matter?
  17. LDBennett

    LDBennett Well-Known Member

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    JPD:

    If you look at the ballistic charts in reloading manuals you will see that they make an assumption that the scope is about 1.5 inches or some other distance above the centerline of the bore.

    I'm looking at the ballistic charts in the Hornady manual, for instance:

    44 mag 0.8 inches
    30 cal rifle 1.5 inches

    You can tell that is the scope mounting height above the bore centerline because the first column of the chart is the drop at the muzzle or zero range.

    Most rifles end up with scope about 1.5 inches above the bore centerline and that is what scope manufacturers base their drop calculation on. The AR is special because the top of the stock is so high in relation to the bore centerline. The stock is that way to accommodate iron sights on the top of the handle of the original M16/AR15. To get the right ergos for a scope on a flat top AR, the scope has to be mounted higher than 1.5 inches and that somewhat effects the ballistics as given by the reloading manuals and the compensating reticules. How much I really don't know but nothing is perfect. Testing is the only way a shooter can determine accurately what the drop is (which slash to use on the reticule for a given distance).

    LDBennett
  18. jpdusn

    jpdusn New Member

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    Thanks LD.
    I've never done any reloading so accessing those manuals has never been a requirement for me and thus never seeing any charts like the one above. I just got to thinking about BDC reticule type scopes, and the geometry due to different scope heights just didn't add up as to how the Mfg's could pull this (one size fits all) off accurately. Apparently it looks a little shady on their part. Some people don't have access to a 200, 300, 400, 500 yard range to verify their reticule and may shoot at targets (animals) strictly based on Mfg's supplied information and that could be a little dangerous.

    I guess the old school way still works the best. Shoot, shoot, shoot and shoot some more. Your rifle, your ammo, your scope, your targets, at different distances if you can. You can't buy shortcuts to often that actually work.
  19. LDBennett

    LDBennett Well-Known Member

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    jpdusn:

    The difference between a scope mounted the standard 1.5 inches off the bore center line and one mounted 2 inches probably is insignificant and swamped out by all the other variable in loaded ammunition whether it be hand loads or factory ammo. Air density effects the ballistics too but that is not variable the manufactures test for. Testing is the only way to be sure. Most ranges have a 200 yds range and one I used to go to had steel targets all the way out to 600 yds and beyond. We shoot our 50 Cal gun (50 BMG based CA legal 50DTC) out to 800 yds and are looking for a spot to get to 1000 yds, which might be doable in the desert where we shoot.

    LDBennett
    Last edited: Jul 12, 2012
  20. jpdusn

    jpdusn New Member

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    Ok, so I did some further research and realized that it is fairly simple. On the above chart, agreeing that the "0" is line of site (center of scope), and the -2.5 is the bore center and/or scope height; a different scope height of lets say 1.5 inches would work like this:

    Using the green line as the reference, if you started another line (lets call it black) on this chart for a scope with a height 1.5 inches this black line would start at -1.5 on the left edge of the chart. Exactly 1 inch above the green line. It remains 1 inch higher throughout the distance of the chart because the path or trajectory of the bullet doesn't change. Where the green line crosses "0" at 50 yards the black line will be an inch high. Where the green line crosses "0" again at 225 yards the black line will again be an inch high at that point. It remains like this until the bullets burn off enough velosity and start falling on a more vertical plane.
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