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

[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