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Drop charts

 
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  #1  
Old 01-04-2013, 12:01 AM
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Join Date: Dec 2007
Location: sebastopol, ca.
Posts: 418
Drop charts

Had a strange experience at the range today.
Working up drop charts for my 6.5x.284. Berger 140 gr. 2930 fps through them chrono during load work up, no chrono today.
Shot a golf ball sized rock at 100 yards to verify zero.
Target at 500 yards.
Dialed in 8.5 minutes from my iPhone ballistics app.
Shot high.
Dialed down to 7.5 MOA
Shot high
Dialed down to 6.5 MOA
Perfect elevation. 3 inches to the left due to wind. Compensated and the next two were 1/2" apart. My deer rifle really shoots.
My question is, to get the drops correct the velocity needs to be 3250 fps. I have never had to change my velocity this much.
Anybody else seen this?
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  #2  
Old 01-04-2013, 06:32 AM
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Join Date: Dec 2011
Location: Vandergrift PA
Posts: 1,198
Re: Drop charts

I have ran into the same thing. I chonod my 300 rum @3200 for my program to hit where i have verified its hitting my adjusted speed is 3385.
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  #3  
Old 01-04-2013, 09:15 PM
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Join Date: Sep 2009
Location: Northwest Ohio
Posts: 224
Re: Drop charts

I have come to learn not to pay any attention to what the chrono says for velocity. I have been using it to get ES and SD. But everyone has their own reasons for using a chrono. Which is fine with me.

Jason
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  #4  
Old 01-04-2013, 09:50 PM
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Join Date: Apr 2009
Location: New Mexico
Posts: 224
Re: Drop charts

I too have seen this from time to time, one thing to check is how your scope is tracking.... Set up a 100yd target, now use a level to extend a perfectly strait line upwards of 30".......put a 1" target at 10", 20", and 30"....shoot the 100 yd zero, dial your scope 10MOA, shoot another ( hopefully you hit the dot 10" above) then repeat for the 20&30 MOA and see where it shoots......i have one scope that tracks 5% off meaning if I dial 10 MOA (40 clicks) I'm high on the 10MOA, if I dial 38 clicks I'm dead nuts......next take this and enter it in as your elevation correction factor in shooter and it will help......forget the .06 MOA....you won't notice .06" at must normal ranges.
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  #5  
Old 01-05-2013, 12:49 AM
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Join Date: Feb 2011
Location: Halfway between Lubbock and Dallas
Posts: 4,633
Re: Drop charts

Quote:
Originally Posted by squirrelduster View Post
Had a strange experience at the range today.
Working up drop charts for my 6.5x.284. Berger 140 gr. 2930 fps through them chrono during load work up, no chrono today.
Shot a golf ball sized rock at 100 yards to verify zero.
Target at 500 yards.
Dialed in 8.5 minutes from my iPhone ballistics app.
Shot high.
Dialed down to 7.5 MOA
Shot high
Dialed down to 6.5 MOA
Perfect elevation. 3 inches to the left due to wind. Compensated and the next two were 1/2" apart. My deer rifle really shoots.
My question is, to get the drops correct the velocity needs to be 3250 fps. I have never had to change my velocity this much.
Anybody else seen this?
What direction was the wind coming from and was it level, rising, or falling ?

If your scope is dead on you might want to try adjusting your BC. Remeber BC's are not static, as velocity drops off so do BC's. The BC's we get from the factory specs are essentially averages based on what ever ranges they are testing the bullets at or based on what a computer program spits out.

Your Chrony, Scope, Rifle may all be dead on but the BC a bit low for the velocity you are running at the muzzle.
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  #6  
Old 01-05-2013, 01:19 AM
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Join Date: Dec 2007
Location: sebastopol, ca.
Posts: 418
Re: Drop charts

Winner, the scope is a Nightforce that has been on multiple rifles with no issues before so I doubt if it has anything to do with the problem.

I am planning on going out again next week with the adjusted velocity and checking different ranges to see if it holds.

I adjusted the bc up to .700 and it didn't make much difference at such a short range.
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  #7  
Old 01-05-2013, 01:40 AM
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Join Date: Feb 2011
Location: Halfway between Lubbock and Dallas
Posts: 4,633
Re: Drop charts

Quote:
Originally Posted by squirrelduster View Post
Winner, the scope is a Nightforce that has been on multiple rifles with no issues before so I doubt if it has anything to do with the problem.

I am planning on going out again next week with the adjusted velocity and checking different ranges to see if it holds.

I adjusted the bc up to .700 and it didn't make much difference at such a short range.
The difference at long range should be more pronounced.

I'd keep tweaking both the BC and velocity until you actual drops at 3 different ranges match up.

I like to use 200, 500, 700yds. If you can get it pretty close at three different ranges then your chart should be pretty well true at any range.
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