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25-04-2019, 12:08 #1
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- Aug 2011
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- Virtutopia
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- 41
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- 9,390
Determining cross hair height above bore
Hey guys,
How do you go about determining the height of your cross hair above your bore for trajectory calculations (like one does on the JBM Ballistics calculators)?
I have been using a standard of 1.6 inches (40.6mm) when I have a 40mm ocular bell.
What is the proper methodology though?
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25-04-2019, 12:16 #2
- Join Date
- Aug 2009
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- 5,600
Re: Determining cross hair height above bore
Following with interest. I use the Meopta App and I'm also not sure how exactly. So far I place my rifle in a vice and use a metsl rular on top op the barrel just in front of the front objective. But my barrel is lower at the crown "If that makes sense"
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25-04-2019, 12:32 #3
- Join Date
- May 2010
- Location
- Right next to the pot that needs stirring.
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- 45
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- 2,157
Re: Determining cross hair height above bore
Is variation of 5-10 mm going to make any practical difference? I believe not. In any case, I put my rifle on the bench, dial to lowest magnification, have another person holding a ruler at the crown and then read of through the scope where the cross hair is marking on the ruler. It does happen that focus is bogus, so then I just use a brightly coloured ruler and move a piece of paper over the ruler until the difference in colour is at the cross hair and the helping hand can then read on the ruler where the paper line is at this stage. Crude, but seriously, that 5-10 mm at the bore has as good as 0 influence on practical shooting.
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25-04-2019, 12:48 #4
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- Aug 2009
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- 5,600
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25-04-2019, 12:50 #5
- Join Date
- Apr 2009
- Location
- Limpopo
- Posts
- 1,051
Re: Determining cross hair height above bore
Measure bolt and ÷ 2, measure scope main tube and ÷2. Measure gap between bottom of scope tube and top of bolt, add results of above and you have height above bore.
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25-04-2019, 13:01 #6
- Join Date
- Aug 2012
- Location
- Pretoria, South Africa
- Age
- 34
- Posts
- 12,547
Re: Determining cross hair height above bore
@killzone - what app is that? I like using Strebor, but I would love to get my hands on some more apps, specially ones with more free features.
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25-04-2019, 13:05 #7
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- Aug 2009
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- 5,600
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25-04-2019, 13:30 #8
- Join Date
- Aug 2012
- Location
- Pretoria, South Africa
- Age
- 34
- Posts
- 12,547
Re: Determining cross hair height above bore
Aww, bummer. Thanks anyway! Back to Strebor I go...
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25-04-2019, 13:35 #9
- Join Date
- Aug 2011
- Location
- Virtutopia
- Age
- 41
- Posts
- 9,390
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25-04-2019, 13:55 #10
- Join Date
- Oct 2015
- Posts
- 114
Re: Determining cross hair height above bore
I just measure the bell of the scope + the barrel in line where I measured the bell divided this by 2 + the gap between bell and barrel also in line where I measured the scope bell and you have your sight hight.
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