A4 - Measuring Centerline of Bore to Top of Rail

[Scorpio64 5,147]

Just mounted a new scope and I'm trying to be a little more "scientific" with this one.  I need to find out what the centerline of bore to top of A4 rail is.  I have found two measurements on-line, one is 1.250 and the other is 1.215.  Can anyone verify which one is the correct measurement.

[T Bill 649]

Always understood to 1.250.  AR15 I presume.

 

http://www.weaponeer.net/forum/uploads/appletech/files/2009-02-27_015736_AR15A3_Upper_COLOR.pdf

[Shocker 150]

If you look at page 8 here http://www.firearmfiles.com/files/AR15/ar15upper_flattop.pdf it looks like

 

1.875-0.625 = 1.250

 

EDIT: looks like the same info as T Bill's. Too slow today...

[Scorpio64 5,147]

If you look at page 8 here http://www.firearmfiles.com/files/AR15/ar15upper_flattop.pdf it looks like

 

1.875-0.625 = 1.250

 

EDIT: looks like the same info as T Bill's. Too slow today...

 

I've seen that pdf.  The problem is, the measurement (1.875) is taken from the outer diameter of the barrel attachment point.  Id need to know the receiver wall thickness, subtract that from 1.875 and then subtract .50 from the one inch diameter of the BAP.  So I'm still unclear about the measurement.

 

Edit: Correction.  1.875 is the measurement from the top of the rail to the bottom of the receiver where it mates with the lower.

[Vlad G 345]

There is a simpler solution to this problem which does a better job of calculating height over bore, IMHO. Take your hand guard off. and measure the total distance between the top of the scope and the bottom of the barrel. Then minus half the diameter of the barrel and half the diameter of the scope as measured at the point you used for the first measurement. You are going to have a number of stacking numbers here, is the receiver perfectly in spec, is the scope mount perfectly measured for its height over rail, etc,   Just measure the assembled product.

[Old School 611]

And what are you doing that you need to know this measurement?

[Vlad G 345]

My guessing about his needs, but for mine knowing the sight line height over bore is useful for ballistic calculations and matching loads to reticles and the like.

[Shocker 150]

I've seen that pdf.  The problem is, the measurement (1.875) is taken from the outer diameter of the barrel attachment point.  Id need to know the receiver wall thickness, subtract that from 1.875 and then subtract .50 from the one inch diameter of the BAP.  So I'm still unclear about the measurement.

 

Edit: Correction.  1.875 is the measurement from the top of the rail to the bottom of the receiver where it mates with the lower.

 

I'm not following you there...the flat bottom of the receiver is the datum for both the center of bore and the top of the rail which is why you can subtract the two and what you're left with is the distance from the top of the rail to the center of the bore.

 

Then if you have a precise rail-to-center measurement of your scope mount, add that and you're GTG.

[Scorpio64 5,147]

My guessing about his needs, but for mine knowing the sight line height over bore is useful for ballistic calculations and matching loads to reticles and the like.

 

BINGO!

 

I just bought a Leupold Mark AR mil/mil scope for my SPR.  Will be learning how to use a ballistics calc and all that jazz.

[Scorpio64 5,147]

I'm not following you there...the flat bottom of the receiver is the datum for both the center of bore and the top of the rail which is why you can subtract the two and what you're left with is the distance from the top of the rail to the center of the bore.

 

Then if you have a precise rail-to-center measurement of your scope mount, add that and you're GTG.

 

There is a lip under the BAP which adds an unknown value.  The 1.250 figure is close, but it does not account for (subtract) the lip under the BAP.

[Eric. 9]

BINGO!

 

I just bought a Leupold Mark AR mil/mil scope for my SPR.  Will be learning how to use a ballistics calc and all that jazz.

Dude, you're over thinking this over .035.  Unless you are shooting ELR, you'll nvr know the difference.

[Old School 611]

My guessing about his needs, but for mine knowing the sight line height over bore is useful for ballistic calculations and matching loads to reticles and the likeOk.

 

OK that is what I do as well. But I don't break the CL of Bore to CL of optics into thousandths.  I try to get as close as a 1/16" and I'm good to go.

Now I've never really thought this through but any error in distance between in the height (let's say 1/8") would be carried through out the trajectory and arc and can be adjusted out by one vertical click of the scope since CL bore to Optics are parallel.  Now if using a 20 or other MOA base I'd have to think about it.

[Scorpio64 5,147]

OK that is what I do as well. But I don't break the CL of Bore to CL of optics into thousandths.  I try to get as close as a 1/16" and I'm good to go.

Now I've never really thought this through but any error in distance between in the height (let's say 1/8") would be carried through out the trajectory and arc and can be adjusted out by one vertical click of the scope since CL bore to Optics are parallel.  Now if using a 20 or other MOA base I'd have to think about it.

 

As it so happens, the Leupold Mk 1 cantilever base plus the Weaver rings do add up to almost exactly 20MOA.  1.044".

 

I know this is splitting hairs to many people but this is how I do things.  Shooting is not just a sport to me, it's also a science.  I enjoy both aspects.

[Eric. 9]

OK that is what I do as well. But I don't break the CL of Bore to CL of optics into thousandths. I try to get as close as a 1/16" and I'm good to go.

Now I've never really thought this through but any error in distance between in the height (let's say 1/8") would be carried through out the trajectory and arc and can be adjusted out by one vertical click of the scope since CL bore to Optics are parallel. Now if using a 20 or other MOA base I'd have to think about it.

The impact of mechanical offset over distance is easily seen with any ballistics program. Im too lazy to verifythe actual numbers but do know from past experience that a miscalculation of, say, 1/16" might equate to about a half inch at 1000yds.

 

*edit- what distance are we talking about? AR shooting 556/223?

[Old School 611]

As it so happens, the Leupold Mk 1 cantilever base plus the Weaver rings do add up to almost exactly 20MOA.  1.044".

 

I know this is splitting hairs to many people but this is how I do things.  Shooting is not just a sport to me, it's also a science.  I enjoy both aspects.

 

Is the base supposed to be 20MOA designed as such or is that your determination?

And it's always a science unless you're a "banger".

[Old School 611]

The impact of mechanical offset over distance is easily seen with any ballistics program. Im too lazy to verifythe actual numbers but do know from past experience that a miscalculation of, say, 1/16" might equate to about a half inch at 1000yds.

 

*edit- what distance are we talking about? AR shooting 556/223?

 

"Mechanical Offset"? Are you saying offset as in MOA or CL bore to CL of optics?

If you shoot distance we all (most) use ballistics calculators.

And 1/2" @ 1000 yds, and I have no idea what you're talking about, equates to .050" at 100yds.

Not even the benchrest guys are that consistent. 

[Vlad G 345]

Now I've never really thought this through but any error in distance between in the height (let's say 1/8") would be carried through out the trajectory and arc and can be adjusted out by one vertical click of the scope since CL bore to Optics are parallel.  Now if using a 20 or other MOA base I'd have to think about it.

 

CL bore to optics are not really parallel, or at least not when zeroed or other wise the bullet would never actually impact were desired. The CL of the physical optics may be parallel with the CL of the bore, but not the line of sight, if that makes sense. 

 

I'm with you that thousands don't matter, the bullet manufacturing variations even in the same batch will make more difference then small distance measurements would ever make. After all whatever the measurement and ballistic calculators say are only guidelines anyway, and you need to verify them as in practice there are always some differences. 

 

To the OP, don't obsesses over small numbers at this point.   You if you can get a decent measurement like 2.7 - 2.75, take that and plug it in with you behavior bullet, adjust velocity numbers until you get a trajectory you like, then try to load your ammo to that velocity THEN see how it groups and how it flies. Alternatively if you have a particular ammo brand in mind, shoot it, chrono it, plug its numbers in and see where that gets you. Absurdly accurate measurements of scope offsets won't matter. 

[Old School 611]

CL bore to optics are not really parallel, or at least not when zeroed or other wise the bullet would never actually impact were desired. The CL of the physical optics may be parallel with the CL of the bore, but not the line of sight, if that makes sense. 

 

I'm with you that thousands don't matter, the bullet manufacturing variations even in the same batch will make more difference then small distance measurements would ever make. After all whatever the measurement and ballistic calculators say are only guidelines anyway, and you need to verify them as in practice there are always some differences. 

 

To the OP, don't obsesses over small numbers at this point.   You if you can get a decent measurement like 2.7 - 2.75, take that and plug it in with you behavior bullet, adjust velocity numbers until you get a trajectory you like, then try to load your ammo to that velocity THEN see how it groups and how it flies. Alternatively if you have a particular ammo brand in mind, shoot it, chrono it, plug its numbers in and see where that gets you. Absurdly accurate measurements of scope offsets won't matter. 

 

Vlad - I'm with you 100%  I was referring to the physical external mounting as you are.  And really that's all we can measure when mounting.

[Eric. 9]

"Mechanical Offset"? Are you saying offset as in MOA or CL bore to CL of optics?

If you shoot distance we all (most) use ballistics calculators.

And 1/2" @ 1000 yds, and I have no idea what you're talking about, equates to .050" at 100yds.

Not even the benchrest guys are that consistent. 

With mechanical offset, I'm referring to the difference between line of sight vs line of bore/bullet path.

 

When I mentioned "1/2" ", I was using a rough example (not an exact calculation).  What I'm getting at is that if you mis-measure the scope bore height by 1/16" (for example), it might only affect the actual bullet impact by around a 1/2", at 1000 yds(for example).

 

The point Im getting at is that unless you shoot ELR, a 1/16" or less miscalculation of scope bore height will be such a small, samll error in the ballistic program that it doesnt warrant any more thought over it.

[Scorpio64 5,147]

After looking into the "20 MOA" bases I cant really say that's what I have, per se.  It appears, if i've read the articles right, that the bases on the 20moa bases are canted to accommodate certain reticles in extreme long distance shooting.  My riser and rings are not canted.

 

The farthest I'll get to shoot for a while will be 300 yards.  I know these tight tolerances i'm aiming for wont make much of a practical difference right now.  tbh, i'm not even that good a marksman yet.  But still, I do love messing with numbers.

 

I will probably get myself a chrono for x-mas.  I'll need/want that for when I get into reloading.  I'm using PPU .223 75gr OTM mostly and according to reviews, the velocity seems to be pretty consistent.

[Eric. 9]

After looking into the "20 MOA" bases I cant really say that's what I have, per se.  It appears, if i've read the articles right, that the bases on the 20moa bases are canted to accommodate certain reticles in extreme long distance shooting.  My riser and rings are not canted.

 

The farthest I'll get to shoot for a while will be 300 yards.  I know these tight tolerances i'm aiming for wont make much of a practical difference right now.  tbh, i'm not even that good a marksman yet.  But still, I do love messing with numbers.

 

I will probably get myself a chrono for x-mas.  I'll need/want that for when I get into reloading.  I'm using PPU .223 75gr OTM mostly and according to reviews, the velocity seems to be pretty consistent.

Here's a great online Ballistics program, in case you were looking for one:

 

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

 

For the sake of argument, a .035 difference in scope bore height calculation amounts to .1 MOA.  At 1000yds, that amounts to about a 1" error.  At 100 yds, that is a 1/10th of 1" error.  Dont sweat the little things, my man.