My rifle has a 40 moa mount and I have always determined bore height from the center of my front ring to the center of the chamber gas port. Is this method correct or should I be measuring from the center of the rear ring to the center of the bore? Thanks
- Thread starter Paul1262
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I think most commonly it’s measured from center of windage turret to center of chamber. At least that’s how I do it.
Thanks.
Yeah just go from the center of the windage turret to the center of the chamber/bolt. Don’t sweat it to much, closest 1/4” will be fine.
Yup center of bolt/bore to center of the windage knob. Doesn't matter if it's a 40 MOA base. Takes about 5 seconds and no match with a ruler or calipers.
Thanks.
I appreciate the extra effort in providing the photos. Thanks.
I've always done it by measuring from the center of the scope ring to the top of the bolt then adding the radius. I measure the diameter of the bolt and divide by 2 to get my radius, then add the radius to the overall length. Should be pretty damn close to dead center that way. 
Just for fun, build yourself a data table based upon today's conditions, then change your Sight Height by .2 and see what that does to your data. It might just be the small tweak you are looking for.
Plug in sight height into a calculator.
An example (AR-15 with a 2.7" scope above bore)
2.7", 85.5, 3000fps, 1000yds:
25.4 MOA drop, 1504fps @ 1000
Just changing to 2.5" bore height,
25.6 MOA drop,, 1504fps @ 1000
2.9" bore height,
25.2 MOA drop, , 1504fps @ 1000
An example (AR-15 with a 2.7" scope above bore)
2.7", 85.5, 3000fps, 1000yds:
25.4 MOA drop, 1504fps @ 1000
Just changing to 2.5" bore height,
25.6 MOA drop,, 1504fps @ 1000
2.9" bore height,
25.2 MOA drop, , 1504fps @ 1000
Similar, measure both diameter of chamber and diameter of scope, add the radius of both, then take easy measurement from bottom of scope to top of barrel and add that number to your diameters# and should be perfect.I've always done it by measuring from the center of the scope ring to the top of the bolt then adding the radius. I measure the diameter of the bolt and divide by 2 to get my radius, then add the radius to the overall length. Should be pretty damn close to dead center that way.
Next up, "Complexities of Pencil Sharpening".
Chapter 1: Carpenter Pencil Sharpening - Shave angle considerations. (Complete with shave angle tables)
Chapter 2: Mechanical Sharpening - Rotational cutting & Speed Equations.
Chapter 1: Carpenter Pencil Sharpening - Shave angle considerations. (Complete with shave angle tables)
Chapter 2: Mechanical Sharpening - Rotational cutting & Speed Equations.
@USMC1990 Well dude....People are laughing at us. Which tells me that I'm either completely wrong and have no idea what I'm doing or I'm correct in a very stupid way worthy of laughter. 
Why the laughs @Gingerman & @Rob01 ? Is my method wrong or correct? I don't want to provide wrong answers to a genuine question.
Why the laughs @Gingerman & @Rob01 ? Is my method wrong or correct? I don't want to provide wrong answers to a genuine question.
@USMC1990 Well dude....People are laughing at us. Which tells me that I'm either completely wrong and have no idea what I'm doing or I'm correct in a very stupid way worthy of laughter.
Why the laughs @Gingerman & @Rob01 ? Is my method wrong or correct? I don't want to provide wrong answers to a genuine question.
The laugh is because you are taking something that is as simple as a 10 seconds with a ruler or caliper and popping it in your ballistic program and making it a college algebra problem with measuring this and that and adding the square route of Pi then dividing by the speed of light. LOL As long as you are within about 1/4 inch you are fine. Does not need to be correct to the 4th decimal point.
Just laughing with you guys. I'm just on a campaign to limit my obsessiveness.@USMC1990 Well dude....People are laughing at us. Which tells me that I'm either completely wrong and have no idea what I'm doing or I'm correct in a very stupid way worthy of laughter.
Why the laughs @Gingerman & @Rob01 ? Is my method wrong or correct? I don't want to provide wrong answers to a genuine question.
I thought the thread was going to be about scope height over barrel and what's best. Then we could delve into barrel heat mirage, scope tube flex in super slo-mo films presented, and such things,
This is a typical question about bore height using a mount with built in MOA (40 MOA example in first post).
Just how accurate do you need to be when you measure bore height?
Is a tenth of an inch good enough?
Using 'arithmetric'
We all know that 1 MOA is about 1.047" @ 100 yds,
or 10.47" at 1000yds.
Dividing 1MOA @ 100yds by 3600 gives us 1MOA @ one inch, or about 0.00029".
How about 40 MOA?
40 MOA is about 41.88" @ 100 yds
or 418.8" at 1000yds,
or 0.01163" per inch.
The original question was where to measure bore height with a 40 MOA mount.
The dimensional slope from rear ring (or front ring) to turret center (about 2 inches) for 40 MOA will be
about 0.02328".
Trignomolygy
Using the tangent function, rise over length, of 40MOA scope tilt (2/3 degree)=0.01163" per each inch of length.
100yds is 3600 inches, 1000yds is 36,000 inches. 40 moa @ 1000yds is about 418.68"
For example; measure scope center over bore height at the turret and get 1.7",
you should get about 1.723" at the rear ring, or 1.677" at the front ring.
Plug those numbers into your favorite ballistic calculator to see what drop @ 1000yds you get using either.
With my earlier example, 85.5 berger, 3000fps, sight height of 1.72" gives 280.8" drop @ 1000 yds, sight height of 1.70" gives 281" drop @ 1000yds,
sight height of 1.68" gives 281.2" drop @ 1000 yds, a total difference of 0.4".
That's about 0.038 MOA.
Then my question, Is a tenth of an inch good enough?
0.1 inch ERROR in bore height measurement (per JBM calculator) is a little less than an inch @ 1000yds.
Lots better than my shooting
Someone mentioned 1/4 inch is good enough.
0.25" error in your scope height measurement will be about 2.3" @ 1000yds.
That's almost 1/2 the 1000 yd F-Open X ring.
Do you want to give away half the X-Ring?
Guess that's what sighters are for
Just how accurate do you need to be when you measure bore height?
Is a tenth of an inch good enough?
Using 'arithmetric'
We all know that 1 MOA is about 1.047" @ 100 yds,
or 10.47" at 1000yds.
Dividing 1MOA @ 100yds by 3600 gives us 1MOA @ one inch, or about 0.00029".
How about 40 MOA?
40 MOA is about 41.88" @ 100 yds
or 418.8" at 1000yds,
or 0.01163" per inch.
The original question was where to measure bore height with a 40 MOA mount.
The dimensional slope from rear ring (or front ring) to turret center (about 2 inches) for 40 MOA will be
about 0.02328".
Trignomolygy
Using the tangent function, rise over length, of 40MOA scope tilt (2/3 degree)=0.01163" per each inch of length.
100yds is 3600 inches, 1000yds is 36,000 inches. 40 moa @ 1000yds is about 418.68"
For example; measure scope center over bore height at the turret and get 1.7",
you should get about 1.723" at the rear ring, or 1.677" at the front ring.
Plug those numbers into your favorite ballistic calculator to see what drop @ 1000yds you get using either.
With my earlier example, 85.5 berger, 3000fps, sight height of 1.72" gives 280.8" drop @ 1000 yds, sight height of 1.70" gives 281" drop @ 1000yds,
sight height of 1.68" gives 281.2" drop @ 1000 yds, a total difference of 0.4".
That's about 0.038 MOA.
Then my question, Is a tenth of an inch good enough?
0.1 inch ERROR in bore height measurement (per JBM calculator) is a little less than an inch @ 1000yds.
Lots better than my shooting
Someone mentioned 1/4 inch is good enough.
0.25" error in your scope height measurement will be about 2.3" @ 1000yds.
That's almost 1/2 the 1000 yd F-Open X ring.
Do you want to give away half the X-Ring?
Guess that's what sighters are for
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Take it easy Francis, measuring diameter and doing simple addition/subtraction is not a college algebra problem. And it also takes about 30 seconds, perhaps you should have stopped pulling Sally’s pigtails and paid more attention in 3rd grade 
Take it easy Francis, measuring diameter and doing simple addition/subtraction is not a college algebra problem. And it also takes about 30 seconds, perhaps you should have stopped pulling Sally’s pigtails and paid more attention in 3rd grade
Have fun with your math class but I will continue to do it the way I have for decades and works and is easy. Guess some like making life harder on themselves. LOL
This is a typical question about bore height using a mount with built in MOA (40 MOA example in first post).
Just how accurate do you need to be when you measure bore height?
Is a tenth of an inch good enough?
We all know that 1 MOA is about 1.047" @ 100 yds,
or 10.47" at 1000yds.
Dividing 1MOA @ 100yds by 3600 gives us 1MOA @ one inch, or about 0.00029".
How about 40 MOA?
40 MOA is about 41.88" @ 100 yds
or 418.8" at 1000yds,
or 0.01163" per inch.
The original question was where to measure bore height with a 40 MOA mount.
The dimensional slope from rear ring (or front ring) to turret center (about 2 inches) for 40 MOA will be
about 0.02328".
Using the tangent function, rise over length, of 40MOA scope tilt (2/3 degree)=0.01163" per each inch of length.
100yds is 3600 inches, 1000yds is 36,000 inches. 40 moa @ 1000yds is about 418.68"
For example; measure scope center over bore height at the turret and get 1.7",
you should get about 1.723" at the rear ring, or 1.677" at the front ring.
Plug those numbers into your favorite ballistic calculator to see what drop @ 1000yds you get using either.
With my earlier example, 85.5 berger, 3000fps, sight height of 1.72" gives 280.8" drop @ 1000 yds, sight height of 1.70" gives 281" drop @ 1000yds,
sight height of 1.68" gives 281.2" drop @ 1000 yds, a total difference of 0.4".
That's about 0.038 MOA.
Then my question, Is a tenth of an inch good enough?
0.1 inch ERROR in bore height measurement (per JBM calculator) is a little less than an inch @ 1000yds.
Lots better than my shooting
Someone mentioned 1/4 inch is good enough.
0.25" error in your scope height measurement will be about 2.3" @ 1000yds.
That's almost 1/2 the 1000 yd F-Open X ring.
Do you want to give away half the X-Ring?
Guess that's what sighters are for
Now that's some math class there. LOL
Not wrong but also not as bad as checking an index pin to make sure it's withing a 0.000002" tolerance.
The bullet is going to exit the muzzle which is probably off center in some random direction from the chamber by some number of tenths or hundredths of inches depending on who made it. Plus, unless it's a 2"+ thick, zero wiggle barrel it's probably going to whip some number of tenths in some random direction depending on the load fired. Similar to what @Paul1262 said, I believe an ambitious level of precision at the point of origin doesn't track all the way to the point of exit. But, I could also easily be wrong even though the ruler method does OK for me.
Curious. Does Chapter 1 get into centering on the graphite core before applying the cutter? If so, I could be interested.
Laugh if you want. Measure with your favorite method. KNOW what your measurements mean.
By the way, one half a mph full value wind is almost 4 inches @ 1000yds
By the way, one half a mph full value wind is almost 4 inches @ 1000yds
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Does you gunsmith measure with a ruler when he blueprints your build?
Is your mount really level to the bore? IS a 40 MOA mount really 40 MOA or 39.5 MOA?
How accurately does your scope track.
Some measurements matter more than others. Scope over bore height just don't matter none.
Shoot, then adjust.
E-Target, Target camera, or experienced Spotter helps.
Is your mount really level to the bore? IS a 40 MOA mount really 40 MOA or 39.5 MOA?
How accurately does your scope track.
Some measurements matter more than others. Scope over bore height just don't matter none.
Shoot, then adjust.
E-Target, Target camera, or experienced Spotter helps.
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Damn it! Now I got to index the firing pin hole to the outside bolt body and mark it for a more accurate measurement.
I can see how being more precise with this measurement would matter more when zeroing on a limited yardage range. Like, if I guy only had 50yds and the BC told him he should be .2 or such low for a 100yd zero.
Ok, too much coffee. I'm gonna go shoot.
I can see how being more precise with this measurement would matter more when zeroing on a limited yardage range. Like, if I guy only had 50yds and the BC told him he should be .2 or such low for a 100yd zero.
Ok, too much coffee. I'm gonna go shoot.
My guess is that with a 40 MOA mount the OP is probably shooting 1000yds or more.
Exactly. You can get within .1" with a ruler or caliper. Some places perfect measurements matter and some don't. Height above bore doesn't and nothing to do with sighters as I shoot a sport with no sighters.
That's where you go off the track in your thinking. It doesn't scare me or others but why do it when it's not needed? Do you do all sorts of things when it's not needed just to drag it out and make it more difficult than it needs to be? Do you use Waze to find your way to the bathroom? LOL I have learned to do spend time on things that need it and not to on things that don't. Nothing about fear but finding my time is more precious than being over anal on something where it's not needed.
This thread would have been a lot shorter and wasted less time reading if I hadn't posted.
First three posts and lock the thread.
Took longer to type the post with the math than to do the math. Time I could have spent watching Free TV
Sorry for adding unnecessary stress.
The OP didn't need to know what impact Scope height or how to measure had on trajectory.
I just doesn't matter.
He owns a Kestral (or equivalent). Just punch in numbers.
Now I DO think that evaluating any measurement for error margin and impact (either yourself or by forum recommendation) and including it or disregarding it is valid. Obvious YOU understand what matters and what don't.
First three posts and lock the thread.
Took longer to type the post with the math than to do the math. Time I could have spent watching Free TV
Sorry for adding unnecessary stress.
The OP didn't need to know what impact Scope height or how to measure had on trajectory.
I just doesn't matter.
He owns a Kestral (or equivalent). Just punch in numbers.
Now I DO think that evaluating any measurement for error margin and impact (either yourself or by forum recommendation) and including it or disregarding it is valid. Obvious YOU understand what matters and what don't.
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Hey guys. This thread did help me. I got way more precise with my measurements. Still no math, just took the measurements sober, and eagle eyed AF!
I thought I was 2” above the bore. Turns out it’s 2.350”. That’s a good enough difference to factor in!
I thought I was 2” above the bore. Turns out it’s 2.350”. That’s a good enough difference to factor in!
Yes doing it sober is definitely a better way. Lol
I’m 52yrs old and brushing up on trig right now, you know, just for fun
Thinking about jumping into Organic Chem next month
Using Waze to get to the bathroom made me chuckle. I understand not wasting a bunch of time doing something that makes little difference, but seriously, I personally like things to be as accurate as possible and like I said, the difference in time is minuscule, just a couple simple measurements. I could have measured the sight height of everyone’s scope in this thread in the time it took me to write this post
You clearly measure once and cut twice if 1/4" of an inch is acceptable.
It's in the realm of doesn't matter but I get mine to .1" quickly with a ruler or caliper but nice try.
Raise your hand if you’re an aspiring pro level F-Class or Benchrest shooter…Ok you guys have the permission to go nerd out over in the corner.
Literally EVERYONE else, gather around over here. Ok so HOB generally isn’t high on the list of importance of gun set up. An error of 1/4 of an inch in HOB is WELL with the repeatable mechanical/fundamental accuracy and precision that 99.9% of shooters can actually deliver on demand.
So if your aspirations are to journey into become rifleman, a master in the craft, whether it is to bang steel or put metal on meat. Then I highly suggest that you start with understanding what you’re prioritizing, because here are about 100 other things that take precedence over becoming an anal retentive about HOB.
It’s not that measuring HOB to the hundredth is going to hurt you directly, it’s just frankly an unnecessary and a distracting side quest. A good factory rifle properly set up to fit a shooter, topped with high end glass, fed quality factory match ammo, all paired with a fundamentally sound shooter will deliver phenomenal precision and accuracy at a rate of fire that will make one of these nerds head spin.
Again, all I am saying is if you starting off into your rifleman journey, you can get a rough measure of your HOB with ruler and then shelf the topic. Don’t get me wrong disciplines like Benchrest or F class do offer a deep look into some of the minutia of rifle shooting which is an aspect of enjoyment but they legitimately have a tendency to get so lost in the mental masturbation of the process.
Literally EVERYONE else, gather around over here. Ok so HOB generally isn’t high on the list of importance of gun set up. An error of 1/4 of an inch in HOB is WELL with the repeatable mechanical/fundamental accuracy and precision that 99.9% of shooters can actually deliver on demand.
So if your aspirations are to journey into become rifleman, a master in the craft, whether it is to bang steel or put metal on meat. Then I highly suggest that you start with understanding what you’re prioritizing, because here are about 100 other things that take precedence over becoming an anal retentive about HOB.
It’s not that measuring HOB to the hundredth is going to hurt you directly, it’s just frankly an unnecessary and a distracting side quest. A good factory rifle properly set up to fit a shooter, topped with high end glass, fed quality factory match ammo, all paired with a fundamentally sound shooter will deliver phenomenal precision and accuracy at a rate of fire that will make one of these nerds head spin.
Again, all I am saying is if you starting off into your rifleman journey, you can get a rough measure of your HOB with ruler and then shelf the topic. Don’t get me wrong disciplines like Benchrest or F class do offer a deep look into some of the minutia of rifle shooting which is an aspect of enjoyment but they legitimately have a tendency to get so lost in the mental masturbation of the process.
As a math nerd, I get my BH super precise using calipers. I open the bolt and measure total distance from top of eyepiece to the bottom of the bolt. I then measure the eyepiece diameter and the bolt diameter. Finally, I subtract half of the eyepiece diameter and half of the bold diameter. That gives you the precise measurement from the center of the scope to the center of the bore.
Better watch out math nerd in that on a base with moa the rear of the scope will be higher than the objective. Oh man your number is not precise to the 4th decimal point!!!!! Better do the objective side and then measure the length of the scope and figure out the center point and then get the average. Lol
What about the height of the reticle in the tube? Does one subtract if it’s below center considering most set the reticle for only 10% down travel?
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