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moa Question

 
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  #1  
Old 12-01-2001, 01:13 PM
 
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moa Question

new to the list
ok i have a question here if a scope is calibrated in moa 1/4 moa = .261 @ 100 yds
then at 1000 yds 1/4 moa would = 2.6179 in a perfect world acording to my calculations
acording to my balistic program shooting 7 mm rem mag 160 gr sierra hpbt @ 2800 fps
mv i should have approx 191 inches drop at 800 yds scope zeroed @ 200 yds so acording to my calculations @ 800 yds 1/4 moa = 2.094" i would need to adjust 91 clicks on the leupold vari x III 6-1/2-20
to achieve a hit not taking into concideration wind also bullet rotational yaw
so my question is am i on base here or am i in left field
Thanks Steve
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  #2  
Old 12-01-2001, 01:53 PM
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Join Date: May 2001
Posts: 2,369
Re: moa Question

Steve

Welcome to Long Range Hunting.

I believe you've got it!! 191 inches of drop at 800 yards = 91.2129894937 clicks on a Leupold scope calibrated at .25 MOA "Clicks". (I'd just go for the 91 "clicks" and skip the .2129894937 [img]images/icons/grin.gif[/img] .)
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  #3  
Old 12-02-2001, 05:25 AM
 
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Re: moa Question

Ok now that we are learning here how about bullet yaw due to rotational spin if the terminoligy is correct the curveture
i guess it would be in y axis is there a formula velocity + barrel twist =
or something like that
thanks
Steve
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  #4  
Old 12-02-2001, 08:06 AM
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Join Date: May 2001
Posts: 2,369
Re: moa Question

Steve

Sounds like you're heading for the 'fuzzy' areas.

There's a bunch of stuff happening to the bullet as it flies. There are 'external' forces, wind, twigs, rain, etc.. there are also 'internal' forces variable on spin, bullet concentricity, jacket thickness variations, center of mass and center of rotation, there is also earth's rotation. It's fairly amazing that we can hit anything with a rifle at any distance greater than an inch or two beyond the muzzle [img]images/icons/grin.gif[/img] .

I believe the one you're heading for is commonly called 'spin drift' or I believe also known as Magnus effect/force. It's the same principle as a throwing a curve ball in baseball.

Yaw as I understand it, is caused by forces acting on the bullets' gyroscopic stability, a low pressure area is trying to lift the front of the bullet as it flies forward while falling, this causes the bullet's nose to deviate 90 degrees from the lift force. I don't believe thhis would cause as much trajectory alteration as 'spin drift' as long as the bullet remains in a non-tumbling state.

There is a lot of information available on the web about these effects and there are some knowledgable folks here too!! It's an area I don't know a lot about and will stay away from any significant technical aspects.

At the distances I shoot and the degree of accuracy I accept and the nature of my targets it's not an immediate issue for me.

There is a book that may cover a good deal of this info, it's authored by a fella that visits this board on occasion, I can only think of his posting name TriggerFifty. I'll look it up and be right back.. Dean Michaelis

Here's a site that has some ingo on Dean, Warren and the long range rifles and shooting.
http://www.cheyennetactical.com/rifles.htm
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  #5  
Old 12-02-2001, 11:00 AM
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Join Date: Nov 2001
Posts: 15
Re: moa Question

The problem with theory is that between us and the long range target there is a great deal of 'real world': conditions that usually cannot be seen, and theory cannot explain, that will move the bullet one way or another. Take a sighter shot, and adjust the equiptment and theories accordingly! [img]images/icons/smile.gif[/img]
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  #6  
Old 12-02-2001, 01:24 PM
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Join Date: May 2001
Posts: 2,369
Re: moa Question

Gwahir

Welcome to Long Range Hunting

Steve

As you can see by the the responses from Gwahir and Darryl the general method of correcting is to fire a spotter or two and correct. This is the method used by most hunters and is a tried and true method.

But, if you're interested in the theory side of the issue(s) there are folks that have a very good understanding of one shot hits at tremendous ranges. These folks don't have the option of firing a spotter round. There are methods to calculate coriolis, angle of slope (even small angles are significant at extreme ranges), spin drift, multivector winds, etc. This is not a better method for the hunter, it's just an alternate method and probably beyond the time and training most folks would would care to invest.
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  #7  
Old 12-02-2001, 09:53 PM
 
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Re: moa Question

ok i was wrong about yaw
what you mentioned about spin drift is what i ment
that is what i am trying to figure out is a drop chart
well i havent even started on the real world stuff the space between here & there
wind rain vectering wind up hill down hill

the rotation of the earth ?
whats that got to do with a 800 to 1000 yrd shot
oh boy looks like im going to get to burn up a few barrels

the sighter shot would probably be the best thing to do
but i doubt if wily will hang around even at a 800 yrd away and 100 yds off of him
maybe im wrong i havent tried it yet

about a half mile is about all i can shoot here i okla. besides the 7rem mag is running out of gas out past that
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