Basic Scope Question

[texasgrunt]

Guys,

I am new to using a scoped rifle. All my previous experience with rifles has been with my grandaddy's pump .22 and my trusty M16A2.

I now have a rifle that is a million times more capable than I am, but I'm working on it. That being said, I have a simple question. I need to figure out the distance between centerlines of my bore and my scope. Should be easy, but I just can't figure out how. I have a Bobro lever mount and a Leupold MK4 ER/T 4.5-14x50 on a VLTOR MUR upper.

All help is greatly appreciated.

Grunt

[bwaites]

Most AR's with scopes are approx. 2.5 inches above the bore, unless tall rings are used.

I'm interested to hear if there is a tool people are using to measure that, beyond the good old MK I mod 0 eyeball!

[texasgrunt]

How close is close enough? If I were to get within 1/8" or so will that be sufficient to run ballistics calculations?

Thanks for the quick response.

Grunt

[mlmiller1]

You can use a micrometer from the center of the bolt, locked open, to the bottom of the scope tube, then measure scope tube & add Half that diameter to get you pretty close. Really, I'm sure within 1/8" is probably close enough. You always have to verify ballistic calculations anyway so do the best you can do with your measurement, then check it in reality.

What I found tricky was with my inclined base mount. HMMMM, I decided from the front of the chamber to the center of the scope was the best I could do & I had to guess at where the front of the chamber is & where the middle of the bore is up & down at that point. I could certainly be wrong though. LOL.

[JASmith]

How close is close enough? If I were to get within 1/8" or so will that be sufficient to run ballistics calculations?

Thanks for the quick response.

Grunt
I lay a carpenter's tape across the scope and barrel and use the Mk1 Mod 0 eyeball to measure.

You can always do a sensitivity test with the ballistics calculator to confirm that the 1/8" or even 1/2" is good enough for your purposes. To do this, take a representative load and run with sigth height of 2.0". Then note the drop at the longest range you are interested in shooting at. Repeat with a scope height of 3.0".

The question to ask yourself: 'Is the difference between 2.0" or 3.0" enough to worry about for your purposes?' I yes, you're done. If not, try a closer range of scope heights.

You may get as much as a half-inch deviation above or below between the muzzle and the zero range. At much greater ranges you'll see larger distances. I did it for the Nosler 120 gr ballistic tip at 2520 ft/sec and a 100 yard zero. The drop at 1000 yards with a 2.0" scope height was -468.8" and with a 3.0" scope height it was -464.3" for a difference of 4.5 inches or ± 2.25 inches assuming 2.5" sight height was nominal and you were off by as much as a half inch in either direction.

That difference at 1000 yards is less than one-quarter moa in elevation. I would not be able to detect that with my shooting skills and equipment. I am usually very happy when I see 1 moa groups!. Others on this forum who can routinely turn in .25 moa groups might see the issue differently.

In any event, the advice mlmiller1 gives "You always have to verify ballistic calculations anyway so do the best you can do with your measurement, then check it in reality." is really top notch. It will correct even greater issues than an error of 1/8" in scope height.

[texasgrunt]

Thanks folks. Looks like I was over estimating the required precision in my measurement.

Grunt

[jwilson1985]

Most AR's with scopes are approx. 2.5 inches above the bore, unless tall rings are used.

I'm interested to hear if there is a tool people are using to measure that, beyond the good old MK I mod 0 eyeball!
what he said

[Mack P]

Grunt,

I had a similar question when I was building my rifle, and I searched ALL OVER trying to find an answer. Best I could come up with was that the height from the bore centerline to the top of the flat-top rail is very close to 1.2" (I actually had an exact number from a blueprint somewhere, but can't find it now). I would assume that most Mil-spec uppers will be the same dimension.

I found a useful chart that has lots of mounts listed, you can get it here:

http://www.maxicon.com/guns/optics/one_piece/ar15_onepiece_mounts.htm

For my combination with a Burris P.E.P.R. and a 1" scope, I came up with 2.7". That seems to work well with my set-up.

One cautionary note; when I was sighting-in, I started with my normal scoped rifle setting of "zeroed at 25 yards", then went to 100 yards, where I would have expected to be 3" to 4" high or so, pending final adjustment. I found that I didn't even hit the paper, until I dropped down a little, where I then had holes at the very top. The extra inch and some in the scope mounting for the AR puts the trajectory a LOT higher at 100 yards than I expected! I had thought that the Marines 37 meters battle sight zero would be close, but the scope mounts are substantially higher than the BUIS sight line.

Mack

[Royal Lancer]

I return my scope to 0:0 then insert a laser bore sighter then measure the laser dot to cross hair center.

[texasgrunt]

Thanks Mack, that's some good data.

Grunt

[Stacyp]

Eyeball it man. This measurement is considered in JBM ballistic calculator but the deviation is very minor. Don't sweat the measurement being exact.

[texasgrunt]

Yeah, eyeball said 2.65 and math says 2.72. I figure .07" is pretty much good enough.

Thanks for the pointers. I'm putting this one to rest.

Grunt