BC, drop tables, math, sights above bore

iwannausername

November 07, 2009 03:06

So I'm trying to get a drop table worked out for a friendly shoot I'm attending at the end of the month.

The online calculator I found drops the BC to 1 digit after the decimal - so it isn't going to be too accurate I don't think. So what is the forumula so I can write my own script to calculate it?

Also, one of the variables to plug in is "how far above the center of the bore are the sights". I'm using a scope - do I measure from middle of bore to middle of scope tube? Objective lens? Ignore it?

Thanks!

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  Hawk Carse

  November 07, 2009 03:42

  BC is not a simple formula unless you already have the form function.
  The BCs used by Quickload are at:
  http://www.shootforum.com/forum/bulletdb.html
  
  Lots of ballistic calculations at:
  http://www.jbmballistics.com/
  
  You can't ignore the sight height, there will be a default value in the program if you don't put in a real measurement. Take the height from the center of your bore to the center of your scope. Lens or tube doesn't matter, they are round and coaxial and it is the same place... unless you have one of the oddball Leupolds with the notch out of the objective end.

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  iwannausername

  November 07, 2009 04:21

  I know the BC of the bullet - .12 - its the 40grn FMJ load for 22wmr used by CCI. I know the published velocity is 1910fps, I'm assuming 1800fps is more accurate.
  
  What I am looking for is a drop table - the contest will be from 25 feet to 225 yards, so I'm trying to figure out where I should zero and then calculate hold over/unders (or change clicks on the scope) for the various ranges. Scope has adjustable focus, so I can see clearly at both extremes...

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