Help with a drop chart....

Colin78

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Jan 25, 2018
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I’m hoping some of you with experience building a drop table can help me out with some questions.

I’m done with load development on 3 rifles so today I shot them out to 300 yards. I know not long range but the longest I had access to right now. All 3 rifles grouped very well, all maintaining less than 1 Moa at 300.

Here are my findings. Muzzle velocity’s were taken with magnetospeed v3.

308. Muzzle velocity 2745 shooting 165 grain Nosler ballistic tip
Zeroed at 100
200. 6 clicks or 1.5 moa drop
300. 16 clicks or 4 moa of drop


6.5 saum muzzle velocity of 3090 shooting 130 grain Sierra tipped game king
Zeroed at 100
200 4 clicks or 1 moa
300 12 clicks or 3 moa


28 nosler muzzle velocity of 3025 shooting 175 grain Nosler Accubond long range
Zeroed at 100
200 4 clicks or 1 moa
300 10 clicks or 2.5 moa

Now here is where my question comes in. When I put my bullet ballistic coefficient and muzzle velocity into the drop calculator my actual findings don’t match the drops listed on the calculator. This is using both the Nikon spot on app and JBMcalculations. In order to make my actual findings match the drops on the calculator I have to increase my muzzle velocity.....quite a bit. Is that the proper way to do it? Changing the velocity or is it best to change the ballistic coefficient?

Could my magneetospeed be this far off?? Here is the change I have to make to get the calculator to match my actual findings.

308 2745 fps to 2850

6.5 saum 3090 fps to 3200

28 nosler from 3025 fps to 3250
 

25WSM

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That's a low scope. This is what I get for your 308
 

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Colin78

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Jan 25, 2018
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Location
Texas Hill Country
Yea that’s what my table shows too. Which is not what is really happening in the field. Change the velocity to 2850 and your table will match what my range findings produced. The 308 was the lest effected. The other 2 were more different.
 

25WSM

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Normally when a customer has issues with his chart he has one of the parameters not correct. Usually scope height. Either they measured it wrong or forgot to enter it. Just double check your numbers you have entered and see.
Shep
 

Colin78

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Jan 25, 2018
Messages
242
Location
Texas Hill Country
My question is, is it best to change the velocity in the program to match the field results or is it better to change the ballistic coefficient? And has anyone seen a magnetospeed that far off? I will say, being that I’m at the upper end of pressure I would tend to believe my actual velocity is higher like the calculators are showing vs what my magnetospeed is showing.
 

Colin78

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Jan 25, 2018
Messages
242
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Texas Hill Country
Normally when a customer has issues with his chart he has one of the parameters not correct. Usually scope height. Either they measured it wrong or forgot to enter it. Just double check your numbers you have entered and see.
Shep

So with the rifle laying on its size I’m holding the calipers in the middle of the scope and the middle of the barrel. Might be a little closer to 1.6” vs 1.5” but at 300 yards that isn’t making much of a difference in the calculator.
 

25WSM

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If your going to start changing things to match up you really need to stretch it out further. I normally find that changing bc a smidge can make the numbers line up pretty good. But you really need to shoot at the longest distance you will be hunting to get the chart right. Fudging numbers at close range can really change your long distance dial ups. If your only hunting to 300 then you already know what your drops are.
Shep
 

25WSM

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You can also verify your velocities with a friend's chrono to see how similar yours is to his.
Shep
 

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