Advanced Marksmanship bore-center to scope-center measurement - VITAL to long range shooting solutions

[blurry6]

One thing that has always kind of bugged me about folks talking about long range shooting (I've drunk some scotch tonight and thought I'd just bring it up) is the importance of SCOPE-TO-BORE HEIGHT measurement and how critical it is to getting accurate ballistic solutions from your android or i-phone software! Plenty of folks talk about the fantastic features of 'ABC' ballistic software and 'XYZ' scope, but never reinforce the fact that having an accurate measurement of scope height in relation to the bore is VITAL to the ballistic solution your high-dollar software spits out. While it might seem to be a 'gimme' element, when asked, I've seen guys kind of guesstimate it on the fly. "Oh, I guess it's an inch and a half"... "I don't know, I measured it with my shell-case, and figured it was 3/4 of 2.8" so about 2.1",...?" Huh? Have you ever run your current load with let's say 1/4" difference in scope height? That'll be a full MIL off at 1000-yards,... a MIL!

I recently helped a guy who had a really nice rig from GAP that shot under 0.3-MOA consistently at 100-yards with his load, but wasn't seeing real-world results beyond 500-yards in accordance with the dope his android software was giving him. He'd chrono'd it and accounted for the inaccuracies inherent in that and started to question the BC on the bullets he'd been using. It wasn't until I asked him if he'd 'REALLY' measured his scope height above the bore did he admit that he hadn't. After measuring it as accurately as possible and dialing it in to his solver, the numbers MIRACULOUSLY fell right into place! While it might not mean anything to the folks that have never taken a shot past 300-yards, it definitely comes into play when trying to reach WAY out there... just a thought. Give me your thoughts.

Ry

 

[Sterling Shooter]

This changes everything; but, anything can change everything. Of greatest importance, yet overlooked or perhaps dismissed by most, is the effect of error associated with inconsistent recoil resistance. A properly kept score book will reveal the source and effect of all errors, whether from the arena of position, principles, or sight adjustment.

 

[angsniper]

What is the best way to get an accurate measurement?

 

[AlliedMarine]

tag for info

 

[milo 2.0]

What is the best way to get an accurate measurement?

Most actions have a vent hole on the ejection port side. Take a caliper and measure center of hole to the middle of front scope ring, should get you.
No hole, you're on your own!
Run a balistics program, it's not as crucial as you think.

 

[blurry6]

I'm no expert on it, and have questions in my own mind regarding canted rings/bases and the effects they might have, but I simply measure from the center of my scope tube, to the center of my barrel diameter,... Whether you use a ruler, calipers, or whatever works best just try to be deliberate in getting an accurate read on it.

 

[oneshot.onehit]

One of the methods

I use a caliper and measure from either the center of the vent hole or even the action Bolt itself to the indicator mark on the scope used for eyeing up your windage hash marks on the turret.


oneshot.onehit

 

[UNCIVILIZED]

1/2 the diameter of your action (assuming it's round) + base height + ring height. With this method, I'm referring to using measurements based on the front of the action, & the thickness of the base in the front, as the rear on many actions isn't necessarily round. Which would goof up your math. Ah, & remember to subtract out for the picatinny lug thickness on the base if necessary.

Or, put in your bore guide & cleaning rod, then measure from the cleaning rod to the underside of your eye piece. After which you do the math based on your eye piece diameter & cleaning rod thickness.

#3 would be to measure from the outside diameter of your barrel, to the underside of your scope (tube, or objective bell). And if you're going to do it this way, put a piece of tape around the barrel where you take your measurement (so you can measure the barrel's diameter accurately after taking the other measurements). That is unless you're measuring where there's no taper to the barrel yet. From here, just add both half of the barrel diameter & half of the scope's diameter to the distance between them & you should be spot on.

I'd figure any of these methods should get you well within 1/10", far less than that if you're real precise about it.

 

[mattj]

Original post is a "bit" of an exaggeration... for example: with my rifle data (.260), a full quarter-inch change in bore height change equates to less than .1 mil change @ 1000 yards. If you have a way to measure it within .1", good for you -- but eyeballing it with a ruler will get you close enough (unless you want to claim that all of your other ballistic calc inputs are so precise that you don't otherwise have to verify your long range dope).

 

[gstaylorg]

My thought is that measuring the height of the optic centerline over bore isn't rocket science. Just measure it. Digital calipers will be better than a wooden yardstick obviously, but it shouldn't be that difficult to find a way to measure the distance between the bore centerline and the scope centerline.

So what is the effect of slight discrepancies in the measurement? I ran two ballistic calculations using the exact same data, which is specifically from one of my .308 setups, except for sight height. The actual sight height is 2.19" as determined using digital calipers. Note in the following two drop charts that decreasing this measurement to 2.0" (essentially 10% low, left panel) only changes the predicted drop at 1000 yd by 0.2 MOA. My actual measured drop at 1000 yd with this setup is 32.8 MOA, so the prediction with the correct sight height is reasonably close.

I can't imagine why someone would be unable to measure their sight height within 10%. Under that level of accuracy, according to this ballistic prediction, you're looking at somewhere in the neighborhood of +/- 0.2 MOA at 1000 yd. Sight height may be more important with some ballistic calculator algorithms than others. However, unless you're a benchrest shooter, there two possibilities that you could actually shoot 0.2 MOA precision at 1000 yd... slim and none. On top of that, the chances are extremely high that wind will play a much greater factor in shifting POI at those ranges. So my take is that of course you want to measure the sight height as accurately as possible, but I think there are other things that have a significantly greater effect on precision at long range than knowing your sight height to 0.01" accuracy.

 

[Sterling Shooter]

Gstaylorg has the proper perspective about this stuff, there are other aspects to LR which are much more vital, yet, are overlooked, or more often, just never stumbled upon by the "long range" internet shooter. Ask the internet long range shooter what's important to long range marksmanship and the answers will have something to do with equipment. Ask the NRA High Master LR Competitor about what's important and the answers will have something to do with the shooter's relationship to the gun and the ground, shot to shot position consistency, holding elevation, or wind strategy. Of course, that kind of stuff is not as interesting to the internet long range shooter who will never need such information, since he has no intent of exposing himself in any sort of long range shooting competition.

The secret for getting the best results at long range, after fundamental knowledge of marksmanship, is about making recoil resistance consistent. That sort of topic here however would not appeal to too many, since it has nothing to do with equipment, other than the by the wayside need to know the rifle must fit the shooter or the shooter must fit the rifle. In other words, that's just too simple. The internet shooter wants a more complex answer, one that can be solved with a purchase rather than something that can only be solved with discipline and practice with the subject.

After winning a mid range tournament at Oakridge many years ago a shooter asked me what my load was. He did not understand my result was not about my load but about holding hard. When I told him this he thought I wanted to protect my secret ingredient ammunition, when what I told him was in fact the secret to good shooting. It was right in his face and he missed it entirely, much like the targets he missed that day.

 

[Greg Langelius *]

Half the bolt body diameter, half the scope tube diameter, added to the distance between the two.

For sloped bases, measure the distance between at the front of the action port and the rear of the action port, and average the two.

This doesn't need to be rocket science. Whatever error derives from the above approach is not really big enough to be comparatively significant.

Yes there may be error and yes, any error is significant, but there is also a hierarchy of priorities at work here. Compared against quite invisible air currents at greater ranges, such an error becomes comparatively insignificant.

This all relates directly to the reasons why I am not so enamored of precision calculations down to the most miniscule decimal. No matter how close you can calculate the value of an element, you have real influence over only a proportion of the elements at work. The ones you don't, the ones you never can, convert all those calculations into close estimates at best.

Calculate less, shoot more.

Greg

 

[Graham]

...having an accurate measurement of scope height in relation to the bore is VITAL to the ballistic solution your high-dollar software spits out. While it might seem to be a 'gimme' element, when asked, I've seen guys kind of guesstimate it on the fly. "Oh, I guess it's an inch and a half"... "I don't know, I measured it with my shell-case, and figured it was 3/4 of 2.8" so about 2.1",...?" Huh? Have you ever run your current load with let's say 1/4" difference in scope height? That'll be a full MIL off at 1000-yards,... a MIL! ...While it might not mean anything to the folks that have never taken a shot past 300-yards, it definitely comes into play when trying to reach WAY out there... just a thought. Give me your thoughts.

You've got it backwards: Scope height is more important inside your zero range than at long distance.

Run the numbers again: 1/4" of scope height equates to about 1/4 MOA at 1000. Can you hold 1/4 MOA at 1000? What's the effect of a 1/2 MPH wind at 1000? So how important is scope height to that shot?

And if your ballistics program is telling you that 1/4" of scope height equals one Mil at 1000 don't use it because it is not accurate.

 

[mk36]

I'm lost...I skipped out of math class to hunt squirrels...

 

[Graham]

I'm lost...I skipped out of math class to hunt squirrels...

Have you learned to add things together?

Your elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume a sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils. Add those two up, and you get about 4.3 MOA or about 1.25 Mils.

 

[TimK]

My software says 3" drop difference at 1K. That's 3" out of 276". Your software ( or the booze) is messing with you.

 

[40xs]

I've never worried too much about the finite distance from axis of bore to axis of scope. As by and large the average distance is close to 1&1/2 inches. with a 100yd. zero and if you know your muzzle vel. most any chart will put you in the black at 500yds and probably at 1000. It helps to shoot comps. then you know EXACTLY what your zeros are at whatever distance. I keep charts for all my loads from 168gr. through 190gr., depending on the distance and how much wind is blowing. I never think about the height of my scope!! Basicaly, it is what it is! keep plugging. ( and getting 2 sighters on each relay helps also)

 

[Graham]

What's a 'sighter' ? LOL!

 

[Rthur]

I'm all for an accurate set of inputs into a bal/cal but DA would also be a good one at 1k.YMMV

R

 

[MAT4-82]

What's a 'sighter' ? LOL!

amen to that

 

[40xs]

Graham, you know! It's what all us old wussies need to make sure we'er on the right target frame. lol

 

[mk36]

Boy am I behind the power curve. I still keep a handwritten logbook on each of my loads based on real testing at the range. I even tape scribbled dope entries to the side of my weapon and cover it with cheap lamination. Probably an embarrassment to all of my fellow digital shooters. I should probably invest in an expensive device to handle all of these chores. Something about those pesky batteries bothers me though...and what if I drop it? I'm pretty clumsy sometimes. If I have some time to spare I might measure bore center to scope center and see what it is. Not sure how I will use it but it might make me sound a bit more knowledgeable when I'm hanging around those high speed guys at the gun shop. Thanks for the info and good shooting!

you talk the talk...do you walk the walk?

 

[Desert_308]

Gstaylorg has the proper perspective about this stuff, there are other aspects to LR which are much more vital, yet, are overlooked, or more often, just never stumbled upon by the "long range" internet shooter. Ask the internet long range shooter what's important to long range marksmanship and the answers will have something to do with equipment. Ask the NRA High Master LR Competitor about what's important and the answers will have something to do with the shooter's relationship to the gun and the ground, shot to shot position consistency, holding elevation, or wind strategy. Of course, that kind of stuff is not as interesting to the internet long range shooter who will never need such information, since he has no intent of exposing himself in any sort of long range shooting competition.

The secret for getting the best results at long range, after fundamental knowledge of marksmanship, is about making recoil resistance consistent. That sort of topic here however would not appeal to too many, since it has nothing to do with equipment, other than the by the wayside need to know the rifle must fit the shooter or the shooter must fit the rifle. In other words, that's just too simple. The internet shooter wants a more complex answer, one that can be solved with a purchase rather than something that can only be solved with discipline and practice with the subject.

After winning a mid range tournament at Oakridge many years ago a shooter asked me what my load was. He did not understand my result was not about my load but about holding hard. When I told him this he thought I wanted to protect my secret ingredient ammunition, when what I told him was in fact the secret to good shooting. It was right in his face and he missed it entirely, much like the targets he missed that day.

"they can't put it on the internet if it isn't true".

Seriously...Sterling, your response is so solid it hurts. It truly is all Indian and no arrow.

 

[Enough Said]

Contributes to cumulative error, but standing alone it is the least of your worries if you are in the ballpark with your measurement.

 

[Graham]

Run the numbers on it: What's the difference on the computer data, for a given load, between using a 1.8" scope height and a 2.5" scope height at 1000 yards? I would bet it is no more than a tenth of elevation, maybe two.

 

[Killer Spade 13]

1/2 diameter of scope tube + 1/2 diameter of bolt + distance between bottom of scope tube and top of bolt.

Or:

(scope tube diameter + bolt diameter)/2 + distance between bottom of scope tube and top of bolt

 

[randy geneske]

I have two questions: Where do you measure? Is it in line with the chamber vent hole, in line with the turrets, in line with where the bullet would be when chambered? These will all be different with a say, 20moa base yes? Ok, the difference is very slight so who cares really, which leads me to my second question.

Sterling Shooter talks about inconsistent recoil resistance, and holding hard: In the prone I try to load the bipod with 1 or 2 pounds of force, and try to pull the rifle straight back with about 4 or 5 pounds of force and the trigger adds another 2 pounds so I guess that makes my recoil resistance between 7 and 9 pounds? I feel this is working ok for me as far as recoil management goes (reticle is not on but very near target when recoil cycle ends) but always searching for more "repeatable" methods. Does "holding hard" mean my recoil resistance numbers should be higher or am I all wet here?

Not trying to derail thread which seemed to have kinda fizzled anyway, just real thirsty for input from the doers, rather than the talkers.

 

[randy geneske]

Forgot to add that I try to relax upper body as I lay into rifle loading the bipod. I do know that some times I don't relax as much, and that would increase recoil resistance and could explain some of the differences I find in group size etc.