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Anti can't scope level recomendations

 
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  #8  
Old 04-30-2013, 05:12 PM
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Join Date: May 2007
Posts: 268
Re: Anti can't scope level recomendations

Quote:
Originally Posted by LouBoyd View Post
I partly agree with what you're saying but it boils down to two things.

For small angles the effect of cant in inches is sine( cant angle) * (192)* T^2 where T is the time of flight of the bullet.. (192 is 1/2 the acceleration of gravity in inches per second squared) This is a horizontal error always perpendicular to the gravity vector. . There is a vertical error component but its still negligible even at 10 degrees of cant By using time of flight the bullet velocity and distance to the target don't matter. The numbers are cant angle versus inches of horizontal error at the target.
T of flight 1 sec 2 sec
1 degree 3.26" 13.0"
10 degree 33.3" 133.3"
Few people hunt game where the range and rifle give 2 second time of flight. Arguably 1 second time of flight is "long range" for most hunters.

But how well can you estimate how canted your rifle is? While humans have the vestibular system of the inner ear which is essentially a set of acceleration sensors that only gives a precision of about 10 degrees for the static angle of the gravity vector. It's only one of several sensors the body uses to sense vertical. The sensors aren't directly perceived by one's consciousness. You only get a feeling of "upright" or "tilting" or "tilted" and that comes from a mix of the available sensors.. When a person is sitting or prone there are better signs of vertical from the forces on whatever parts of their body are supported if a person has no visual visual. With a little practice a person standing may be able estimate vertical without fisual clues to one degree from the the pressure on their feet if there are no other forces than gravity on the body, but light wind pressing on the body can add considerable error.
.
Visual clues are by far the strongest in determine a person's sense of vertical. Being able to see a horizon, standing trees, human made structures, and bodies of water are very strong clues which override other senses even if they're wrong. In mountainous terrain visual clues are often nonexistent or and sometimes present but wrong.

You may not need a level for the distances or the terrain where you shoot. On the other hand it's an error source which can be completely eliminated by using a simple bubble level. It's cheap and easy to to use compared to other sensors like rangefinders and air density meters. If someone would market an instrument which could measure >downrange< crosswinds and calculate the resulting bullet deflection the only error sources left which cannot be easily controlled would be velocity variation of the ammo and the stability of the shooter to hold the rifle steady while pulling the trigger.
Some good points there, but you math is wrong.
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  #9  
Old 05-01-2013, 01:57 AM
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Join Date: Oct 2007
Location: Patagonia Mountains, Arizona
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Re: Anti can't scope level recomendations

Quote:
Originally Posted by X-man View Post
Some good points there, but you math is wrong.
I'm here to discuss and learn. Please post the correct math.
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  #10  
Old 05-01-2013, 02:52 AM
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Join Date: May 2007
Posts: 268
Re: Anti can't scope level recomendations

JBM - Calculations - Trajectory

I suck at math. Brian Litz and his buddy (don't recall his name) made it simple.
6th column on the right is cant angle.
Plug in you sight height and key ballistics and the calculator does the rest.
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  #11  
Old 05-01-2013, 03:39 AM
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Join Date: Dec 2001
Location: Mukilteo, WA
Posts: 1,091
Re: Anti can't scope level recomendations

His math is fine. That calculator is deceptive--it's not taking into account the scope's adjustments being canted with respect to gravity, only the horizontal offset of the scope (not being directly above the bore). The errors from the latter are negligible in the real world, the errors from the former are large and miss-inducing.
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  #12  
Old 05-01-2013, 12:08 PM
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Join Date: Oct 2007
Location: Patagonia Mountains, Arizona
Posts: 758
Re: Anti can't scope level recomendations

Quote:
Originally Posted by Jon A View Post
His math is fine. That calculator is deceptive--it's not taking into account the scope's adjustments being canted with respect to gravity, only the horizontal offset of the scope (not being directly above the bore). The errors from the latter are negligible in the real world, the errors from the former are large and miss-inducing.
For those who read my post I was not trying to provide scope corrections for practical shooting. The previous post to mine was arguing that most shooters don't need to worry about cant. But this is the LongRangeHunting site Admitedly you don't need precise cant measuremnts if to shoot deer at 400 yards.

My post was an attempt to show in what situations you need to corrct for cant. If you choose to measure the cant angle of your rifle and know the ballistics of the bullet from your rifle then the calculations from the JBM site or from Brian's book and ballistics program are the "correct" math.

My post was mainly to point out that the environment is important to how well a human can can estimate cant angle. The math was to show that error for small angles inreases rougly linearly with the cant angle for small angles and fairly accurately increases with the square of the time of flight and that the bullet or distance don't change that. It was not my intent that anyone should use the equation I posted other than to recognize when they need a level. That still depends on the magnitude of error one is willing to accept from uncorrected cant.

So to say my math is incorrect is true if you're trying to use it as a calcaulation for leaving the rifle canted and still hitting the target accurately but why would anyone do that?

Accurately calculating the magnitude of cant can be an entertinaing exercize but it's useless for practical shooting. You have to know the cant angle accurately to do accurate calcualtions. If you know the cant angle just rotate the rifle to eliminate it instead.
A bubble level with it's tube mounted mutually perpendicular to both the rifle bore and the gravity vector when the rifle is "upright" makes that very easy with no calculations. The presicion of a small bubble level, even a cheap one, indcates the zero cant condition better than human can estimate it unless there are other precise and accurate visual clues within their immediate field of view. That's rarely true in any terrain. Maybe at a a well constructed target range.
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  #13  
Old 05-01-2013, 12:15 PM
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Join Date: Oct 2011
Location: Western Pa.
Posts: 174
Re: Anti can't scope level recomendations

Accuracy 1st makes what I use. SUPER rugged an compact. I have one on a 300RUM and one on a Grendel gas gun. Can take HARD field use and tac type training and never let me down.
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  #14  
Old 05-01-2013, 02:12 PM
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Join Date: May 2007
Posts: 268
Re: Anti can't scope level recomendations

Quote:
Originally Posted by LouBoyd View Post
I'm here to discuss and learn. Please post the correct math.

I thought you were suggesting a 133" correction for a 1 second TOF and a 10 degree cant.
I went back and re read you post and discovered I had miss read your table.
I wasn't trying to be a jerk...Sometimes it just happens.
My apologies!

FWIW several years ago I was one of the shooters over at Snipershide asking Brian Litz for a correction table/factor for rifle cant that actually matched our field data. During that time I spent months with an angle finder taped to my stock and though math isn't my thing I do understand the end values for a canted rifle. Brian solved the problem like it was nothing.

That said this is Long Range Hunting and though I draw no fixed line I try to stay inside 600 yards. Once I cross 600 yards I know things start to get complicated in a big hurry. At 1000 yards the variables (for me) are outside of my personal envelope of a sure kill. Out here your 10 degree cant value of 6 inches is noteworthy, however a 10mph wind has 10 times that value. Also four MPH wind error has 4 times the consequence of a 10 degree cant. To put it further into perspective a ten degree rifle cant has the same value at 1000 yards as does spin drift.....Clearly you understand all of this, and have already said as much, but if a guy is asking if he needs a level to hunt with....I don't believe he does.

Mikecr's Pennsylvania post got me thinking that I should also point out that I am not a prairie shooter and that I shoot in a heavily treed stands of Douglas Fir (tall straight and level and the crooked ones stand out). So I am surrounded by level points that (for me) are plenty accurate out beyond 1000 yards.
If going by what Mike says and I went deer hunting in Pennsylvania and I could actually cant my rifle 20 degrees...then at my self prescribed 600-700 yards I have a significant error of 8 inches or so.
In that environment I concede you may need a level.
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