Rifle Scopes Scope hight above barrel

[1ShotJake]

I hope that this is not too dumb of a question...
I know that we should try to mount the scope as close to the barrel as possible, but what is the technical answer.

Thanks in advance,
Rob

 

[sierra2]

Re: Scope hight above barrel

You're right, it it should be mounted as close to the barrel as possible, but it also needs to be at a proper height for the shooter's eye alignment...and that will vary shooter to shooter. Too high can be fixed with a cheek pad. Too low can't.

It has to do with angles, and the farther the scope is away from the center of the bore, the more extreme the angle for point of aim/point of impact...particularly at closer ranges. A GREAT example is our FNH P-90's (which we rarely carry now). The Eotech's are mounted on the rail, above the magazine, which puts the center of the reticle close to 5" above the center of the bore. Entry wanted them zeroed at 7yds for CQB. That's a real steep angle for POA/POI at 7yds. What we found though, was at 50 yards, POI was roughly 30"+ <span style="text-decoration: underline">ABOVE</span> POA. Great for CQB, but it sucked if you encountered a bad guy while approaching the structure!

I don't know if this was the "technical" answer you wanted, but I hope this example helps you.

Eric

 

[Lindy]

Re: Scope hight above barrel

Eric: Whoever wanted them mounted that way was not thinking clearly.

A 25 yard battle zero would have been much better, which would have gotten you something much more useful, pretty much flat for a center-mass hit from 25 to 300 yards.

Then shoot it inside 25 yards to see how high you have to hold on a target to hit it - which cannot exceed the optic height above the bore.

As to the original question, here's the technical part.

There are two angular components to a short-range - say, 100 yard - zero.

One is compensation for the height of the scope over the bore. That angle may be computed as:

angle = arctan (height above bore / distance to target)

In the case of my AI, for example, with a height above bore of 1.75 inches, at 100 yards (3600 inches) it's:

angle = arctan(1.75 / 3600) = .03 degrees or 1.67 MOA

The other component is the drop from the muzzle to the zero range. With my load at 100 yards, that's about 2.7 inches, or, using the same formula above, about 2.58 MOA.

Add those two up, and it's about 4.25 MOA or about 1.25 mil.

Which is probably more than anyone really wanted to know. But that's the amount of elevation which is consumed by a short range zero, which the interested readers, if there are any, can compute for their own setup.

 

[mdmp5]

Re: Scope hight above barrel

It's all about the cheekweld, limited by the stock you are using. For the majority of bolt action rifles with stock combs that are below the place of the top of the barrel, the cheekweld is very nice when the scope is very low. A low scope also enables the line of sight thru the scope to be very close to the line of sight thru the barrel. Lindy gave an excellent explanation on how to quantify it better than I can. However, on an AR for example, the comb of the stock is pretty much in line with the receiver and the barrel, so a super low mounted optic will be too low. So you can't generalize and say that the scope should always be mounted as low as possible.

 

[orkan]

Re: Scope hight above barrel

I think you CAN generalize and say "as low as possible" simply because if you mount it too low for adequate cheek weld, then it "isn't possible."

 

[mdmp5]

Re: Scope hight above barrel

Ok, if you want to delve into semantics, then the statement holds water. However, it is an ambiguous statement.

 

[dreamlander]

Re: Scope hight above barrel

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: palmik</div><div class="ubbcode-body">It's all about the cheekweld, limited by the stock you are using.

</div></div>

Exactly. Having the most comfortable cheekweld out weighs any other variables when mounting the scope. I mounted an IOR on my RRA varmint and put a SRS sniper stock on it. I love the cheek rest and the monopod, but couldn't get low enough on the scope, so I had to get higher mounts and now the scope line in almost 3" above the bore. Makes no difference.
But yeah, the mathematical explanation explains it very well.

 

[EscapeVelocity]

Re: Scope hight above barrel

Thanks for the equation, although I may be missing something (mostly the case when dealing with math), your dividing your scope from your bore(1.75") and dividing it into the distance (3600")? I am a newbie with the same questions and concerns on scope height. My first long range setup was this past weekend when mounting optics on my 700P. Scope height from the barrel is .75", and was concerned this would be a problem at greater distances (angles). Overly concerned I'm sure.

 

[Lindy]

Re: Scope hight above barrel

I'm not sure exactly what you're asking, but basically scope height above bore is an issue at relatively short ranges, and matters less at longer ranges.

Ballistic programs are good experimental tools. Set up your load in one, and then experiment with changing the scope height above bore to see what effect is has at various ranges.

 

[dreever]

Re: Scope hight above barrel

Other than cheek weld can a scope be mounted to low? For example on my GAP Hospitaller using Seekins high rings and a 56mm objective Nightforce the scope bell is VERY VERY close to the barrel. So close in fact that the Nightforce bikini covers won't go on and Butler Creeks just go on and are touching the barrel.
So aside from cheek weld can being this low have an effect on the scope from barrel harmonics, barrel heat mirage, etc. Or dosen't it matter?
I suppose I'm asking is there an ideal height or gap between the scope objective and the barrel?

Danny

 

[Lindy]

Re: Scope hight above barrel

If the Butler Creek cover for the objective lens is touching the barrel, it would be better to eliminate that problem.

 

[Shady_Jay]

Re: Scope hight above barrel

Put the cap on a belt sander to remove a few thousandths. I had to do that to get my PH to fit with high rings and a HV contour barrel.

 

[SickSigma]

Re: Scope hight above barrel

This theory of scope mount is bothering me..consider the following:

A milspec AR with a standard 6X scope (barrel length, bullet spec, etc not important) mounted 1" off the barrel center is sighted in for 100 yards.

The another AR (identical to the 1st one mentioned) fitted with a scope that is, for arguements sake, 2" above center bore. It is again sighted in 100 yards.

Let's move the target now out to 300yds. Both rifles shoot at the bullseye without altering POA or adjusting the scope - which one shoots closer to the center of the target? Neither - Given that all variables, (windage, temp, bullet, barrels, blah blah blah) are equal, they will both shoot in the same spot BECAUSE the angle required of the <span style="font-style: italic">barrel</span> (to make the original 100yd shart) hasn't changed - only the height of the scope tube has, which, while used for aiming purposes, does not dictate the physical characteristics (angle of elevation, ballistics, yardage, etc) required for the bullet to hit the target. Consider the process of sighting in a scope from a bench - the [scope] reticle is moved to where the GUN put the bullet so that the view through the scope MATCHES the ballistics of the gun, not the other way around.

SO, the only good reasons for mounting a scope close to the center of the barrel is balance, and the limitations of the range of the lense inside the scope tube for adjustment purposes.
Agree or Disagree?????


I guess I could also be 1/2 crazy.....