dope for a dope

obilly

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To kick things off; The wind dope math I use is:
(range/100) x wind speed divided by my range constant (12) = moa wind hold

My question is twofold
First: is there a better way to come up with the range constant than by feel? 12 isn't quite right and it’s too expensive for me to feel out the right number.
Second: my reticle and turrets are in mills. Is there a better equation than converting my MOA dope to mills? It’s getting old quick.
 
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LapuaBob

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The equation you are using for wind comes from a simplified equation that fixes wind for a M40 shooting 7.62 special ball. It is not a perfect fit for every rifle. And you may be using it wrong, if memory serves the "constant" changes with range.

Here is an equation that will work for every bullet.
17.6×effective wind speed mph × your bullets lag time= Wind deflection in inches.
Lag time =your bullets vacuum time if flight - its real time of flight.
~ Thanks to Brian Litz, Author of Applied Ballistics for Long Range Shooting; Chapter 5- pages 55 through 58.~

I think your equation comes out of the above somehow but I couldn't see it this morning.

Wind is a liner ballistic force. If a 5mph wind deflects the bullet 1 inch than a 10mph will be 2 inch and 20 is 4.

I like doing the math in the field to get a shooting solution. Wind is always changing so I think you should keep it in numbers you can do in your head.

I have mil values for a 10mph wind written down for each 100 yds. Then I just break it down to quarters. Example: 1 mil @ 10mph. Wind is 7mph today so hold is .75 mil, its blowing in from 1 o'clock so I call it half value. Half of .75 is between .3 and .4 I figure the bullet will land in there and send it before the wind changes.

I use holdover for wind rather than dial because as soon as I get wind dialed in the wind shifts. I recalculate and redial wind and then it changes, I redial and it shifts. Before to long I can't remember shit and miss.

Hope this helps.
 

obilly

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The equation you are using for wind comes from a simplified equation that fixes wind for a M40 shooting 7.62 special ball. It is not a perfect fit for every rifle. And you may be using it wrong, if memory serves the "constant" changes with range.

Here is an equation that will work for every bullet.
17.6×effective wind speed mph × your bullets lag time= Wind deflection in inches.
Lag time =your bullets vacuum time if flight - its real time of flight.
~ Thanks to Brian Litz, Author of Applied Ballistics for Long Range Shooting; Chapter 5- pages 55 through 58.~

I think your equation comes out of the above somehow but I couldn't see it this morning.

Wind is a liner ballistic force. If a 5mph wind deflects the bullet 1 inch than a 10mph will be 2 inch and 20 is 4.

I like doing the math in the field to get a shooting solution. Wind is always changing so I think you should keep it in numbers you can do in your head.

I have mil values for a 10mph wind written down for each 100 yds. Then I just break it down to quarters. Example: 1 mil @ 10mph. Wind is 7mph today so hold is .75 mil, its blowing in from 1 o'clock so I call it half value. Half of .75 is between .3 and .4 I figure the bullet will land in there and send it before the wind changes.

I use holdover for wind rather than dial because as soon as I get wind dialed in the wind shifts. I recalculate and redial wind and then it changes, I redial and it shifts. Before to long I can't remember shit and miss.

Hope this helps.



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Thank you for the knowledge. Your right, my equation is for a 168g sierra BTHP fired @ 2770 FPS and now I’m shooting a 180 sirocco @ 3990. That’s why I’m struggling. You’re also right about the range constant changing. 15 is for 100-500Y. Needless to say, going through all the RCs at all the ranges is out of the question.
To confirm I understand what you’re telling me: If I have a half value 10 MPH wind and a calculated flight time of 1 second (1000 FPS for the entire distance of 1000 yards for ease of math) then I get
17.6 x 5 x 1 = 88 inches
88 inches/ 100= .88 moa wind hold over. In my scope I would hold a quarter mil?

Or should I use a more realistic flight that accounts for velocity drop according to a ballistics chart?

Also, how does BC and bullet cross-section play in? Or is it negligible compared to flight time?

Thanks again for your time.
 

LapuaBob

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As far as the equation goes, its 17.6 × wind mph × Lag time = deflection in inches.
Lag time is not flight time. 3000fps over the entire 1000yds would be representative of vacuum flight time. ( the bullet doesn't slow down because of no air) this flight time is 1 second. Your bullets real flight time is let's say 1.5 seconds. The lag time is the difference between the two, half a second.
17.6 × 5 × .5= 44

The lag time will increase with distance as your bullet decelerates. So it accounts for range. Farther out slower bullet more wind deflection. Also your bullets BC affects deceleration. A higher BC decelerates less over distance and gets a lower lag time. Less wind deflection.
 

LapuaBob

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You can also skip the moa conversion to mils. 1 mil is 3.6 inches at 100yds.
So 3.6 × range in hundreds of yards = inches per mil at distance.

1000yds is 10×3.6=36 inches.

Just divide the wind inches by inches per mil at distance and you get mil holdover.

44 inches ÷ 36 = 1.22 mil holdover.

The less times things get converted the less chance for a mistake. Look at your inch to moa above.

88 inches at 1000 yds is 8.8 moa or 2.44 mil.
 

Darkside-Six

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    We used the law of constants in the Corps back in the mid 90's but soon after found that the Range minus 1 worked better or at least more consistent.

    (Range in 100's - 1) x wind speed (full value) in tenths. or divided by ten.

    example. you're shooting at 500yds with a 10mph left to right cross wind.

    5-1=4, 4x1=4

    or

    5-1=4, 4x10=40. 40/10=4
     

    obilly

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    We used the law of constants in the Corps back in the mid 90's but soon after found that the Range minus 1 worked better or at least more consistent.

    (Range in 100's - 1) x wind speed (full value) in tenths. or divided by ten.

    example. you're shooting at 500yds with a 10mph left to right cross wind.

    5-1=4, 4x1=4

    or

    5-1=4, 4x10=40. 40/10=4

    4 is moa hold over?
     

    29aholic

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    The range minus 1 formula only works for minutes.


    Sent from my iPhone using Tapatalk

    I was trying to break it down for the 100 yard shot. I know going beyond that the math gets a little more difficult. If 1 MOA is 1" at 100 yards and 1 MIL=3.6" then wouldn't it be about 3.96 MILS? The main reason I ask is my reticle is MILS but my knobs are MOA. I prefer to hold off versus dialing.
     

    Darkside-Six

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    I was trying to break it down for the 100 yard shot. I know going beyond that the math gets a little more difficult. If 1 MOA is 1" at 100 yards and 1 MIL=3.6" then wouldn't it be about 3.96 MILS? The main reason I ask is my reticle is MILS but my knobs are MOA. I prefer to hold off versus dialing.

    Ah, that's old school. Well, even if you have MOA turrets with a mil reticle, you can still correct your misses with the right hold.

    There is another formula out there for Mils. it's consistent enough to get you on target. If you've ever seen the Magpul art of the precision rifles DVD's then Todd Hoddnett gives a pretty good break down of it.

    in a nutshell, you break down wind speeds in increments of 4mph. every 4mph is 1 unit. you basically take the range of your target and move the decimal point in front of it and then multiply by how many units of wind you have.

    Example.

    you're shooting 500yds. you would start out with .5
    if your wind is 4ph then that is 1 unit so 1x.5= .5 it's a .5mil hold.

    if it's an 8mph wind then it would be 2x.5= 1mil hold
    6mph would be 1.5x.5= .75mil

    in the long run, the best thing you can do is shoot your rifle and record the information. Everybody's rifle dopes different.
     

    obilly

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    Ah, that's old school. Well, even if you have MOA turrets with a mil reticle, you can still correct your misses with the right hold.

    There is another formula out there for Mils. it's consistent enough to get you on target. If you've ever seen the Magpul art of the precision rifles DVD's then Todd Hoddnett gives a pretty good break down of it.

    in a nutshell, you break down wind speeds in increments of 4mph. every 4mph is 1 unit. you basically take the range of your target and move the decimal point in front of it and then multiply by how many units of wind you have.

    Example.

    you're shooting 500yds. you would start out with .5
    if your wind is 4ph then that is 1 unit so 1x.5= .5 it's a .5mil hold.

    if it's an 8mph wind then it would be 2x.5= 1mil hold
    6mph would be 1.5x.5= .75mil

    in the long run, the best thing you can do is shoot your rifle and record the information. Everybody's rifle dopes different.

    Thanks that seams pretty straight forward. I will have to give it a try. I like that it dopes in mils so I don't need to convert anything. Everything was so much simpler when everyone around me used the same system.
     

    obilly

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    I was trying to break it down for the 100 yard shot. I know going beyond that the math gets a little more difficult. If 1 MOA is 1" at 100 yards and 1 MIL=3.6" then wouldn't it be about 3.96 MILS? The main reason I ask is my reticle is MILS but my knobs are MOA. I prefer to hold off versus dialing.

    Lapua-Bob posted a great formula to convert inches to mils so that's where I would start if I were you. Just remember not to convert inches to MOA first. Not that I'm one to be giving advice. I feel your pain though, I just switched from using a scope like that. With practice I got fast at dialing but never stopped hating it.
     

    29aholic

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    Lapua-Bob posted a great formula to convert inches to mils so that's where I would start if I were you. Just remember not to convert inches to MOA first. Not that I'm one to be giving advice. I feel your pain though, I just switched from using a scope like that. With practice I got fast at dialing but never stopped hating it.

    With darkside 6's mil formula I am good. I hold off for wind and dial for elevation. I know the MOA of my elevation DOPE and dial by the turrets but now I can use the MIL formula for my wind and use the reticle for that.

    I love this place.
     

    Gimp

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    Ah, that's old school. Well, even if you have MOA turrets with a mil reticle, you can still correct your misses with the right hold.

    There is another formula out there for Mils. it's consistent enough to get you on target. If you've ever seen the Magpul art of the precision rifles DVD's then Todd Hoddnett gives a pretty good break down of it.

    in a nutshell, you break down wind speeds in increments of 4mph. every 4mph is 1 unit. you basically take the range of your target and move the decimal point in front of it and then multiply by how many units of wind you have.

    Example.

    you're shooting 500yds. you would start out with .5
    if your wind is 4ph then that is 1 unit so 1x.5= .5 it's a .5mil hold.

    if it's an 8mph wind then it would be 2x.5= 1mil hold
    6mph would be 1.5x.5= .75mil

    in the long run, the best thing you can do is shoot your rifle and record the information. Everybody's rifle dopes different.

    Don't know about the rest of you but I like to have equations for everything.

    [Range(yds)/1000] x [wind speed(mph)/4] = mil hold over
     

    Darkside-Six

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    Don't know about the rest of you but I like to have equations for everything.

    [Range(yds)/1000] x [wind speed(mph)/4] = mil hold over

    Whatever works for you. I don't like having to do math in the field. I'm only speaking for myself here but for me if I have an 8mph wind at 500yds it's much easier to just go .5x2=1 rather than go 500/1000=.5x8=4/4=1 but too each their own. as long as you come up with the proper solution in the end then that is all that matters. :)