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Ballistic Confusion.

 
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  #1  
Old 09-14-2012, 11:39 PM
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Ballistic Confusion.

Took a couple rifles to the range today and something happened that I can't explain. The first is a custom 7 WSM with a Badger 20 MOA base and rings holding a 8-32X56 NXS pushing a 162gr AMAX @ 3034 fps. The range is flat out to 400 yards. The shooting bench is 7 feet off the ground making the barrel right at 8 feet off the ground when on the rest. The targets are about the same height, 8' 6", and are set up at 100, 200, 300 and 400 yds. The rifle is zeroed @ 400 yards. First shot @ 400, dead on. Then I move to 100, shoots 7" high. Move to 200, 11" high. 300 comes in at 11 3/8" high. How can this be? Does not follow the Hornady Ballistic program. The second rifle is a custom 300 RUM launching a 210gr JLK @ 3100 fps. Same base and rings, zeroed at 400. 5" high at 100, 9 1/4" high at 200 and 9 1/2" high at 300. My problem is I just can't understand how increasing the distance from the rifles raises POI and then the last 100 yards, both bullets drop so dramatically. What am I missing here?
db
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Old 09-15-2012, 08:15 AM
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Re: Ballistic Confusion.

The 300yd POIs don't make any sense, and the only way 100/200 does is to enter MV in the 2600s for that bullet.
Are you basing your zero POA on center crosshair or some hash hold-off?
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Old 09-15-2012, 09:31 AM
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Re: Ballistic Confusion.

Confusing isn't it. Just don't know what I'm doing to cause these results. All aiming is done with the center crosshairs. One thing I need to do is measure the angle of the bore and see how far above line of sight I am.
db
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Old 09-15-2012, 10:43 AM
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Re: Ballistic Confusion.

Hmmmm, ran some JBM calcs. Note scope height. Figured is pretty close to mine as scope objective is the same and same brand of scope. The 300 yd value is ~3" too much. Should be around 8?

Here's the JBM inputs I used and results. Drop measurements are very critical. 5 or more shots @ 300 may be necessary???

JBM Inputs


JBM Drop Table


If what you are getting is what your are getting, here's an equation that is good enough to hit what you're shooting at.

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  #5  
Old 09-15-2012, 12:01 PM
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Re: Ballistic Confusion.

The center of the scope on both rifles is 2" above bore and the elevation on both are adjusted to 3 and change. True, I fired only one round on each target, but the consistant accuracy on both rifles is so good and the rest is so solid that I don't believe there are fliers involved. The Hornady program came in at 8.6" @ 200 and 6.9 @ 300 for the 7. I didn't measure the wind, but it was very light, <2. The only thing I haven't done yet is measure the height of the target @ 400. From the bench it looks as though it's about the same as the others, but even if a couple feet higher, that wouldn't account for the increase in POI from 2 to 300 yards, would it?
db
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