Sanity test, Scope adjustment compensation.

Discussion in 'Long Range Hunting & Shooting' started by Greyfox, Nov 7, 2012.

  1. Greyfox

    Greyfox Well-Known Member

    Messages:
    3,711
    Joined:
    Jan 21, 2008
    Testing my Mark 4 for click value/turret adjustment accuracy I found the each click value is actually .262 MOA as opposed to .25 MOA. After going through some math and checks on my Ballistics Calculator I found that I could use my MOA turrets with MOA entered into the BC and replicate my drops out to at leat 1000 yards by reducing my velocity entry by 50 FPS. Better yet I could as an alternative use the IPHY entry in my calculator since the difference is 1.048" per click, since .25"x1.047= .262. This replicated the drop in inches at 1000 yards. The IPHY difference to MOA is 1.047 which puts the actual drop within 1" at 1000 yards. Coincidence? My load is .338 Lapua, 300 Berger, actual velocity 2700 FPS. I will do an actual drop test to confirm this but I wanted to sanity test my logic. Does this make sense?
     
  2. Scot E

    Scot E Well-Known Member

    Messages:
    1,312
    Joined:
    Mar 1, 2008
    I may be reading your info wrong but I think you are making it harder than it is.

    1 MOA = 1.047 inches per 100 yards, so each MOA click is .262 inches, assuming the scope is calibrated correctly.
    1 IPHY = 1.0 inch per 100 yards, so each IPHY click is .25 inches, assuming the scope is calibrated correctly.

    So if your turrets click adjustment moves .262 per click then it is actually moving what it should for true MOA.

    So to test this you can either shoot to find drops or just measure the change in the reticle as you move your turrets. I prefer to just measure the movement as it saves me shooting ammo and keeps shooter error out of the equation. Set up your target, for me a tall piece of cardboard, at exactly 100 yds. Buy or print out 1/4 inch graph paper, make sure it measures true, and piece enough together to get 30 inches. You can buy it large enough but if I print it I have to piece it together. Mark your start point on the graph paper so you don't loose it and then dial in say 20 MOA. Then measure the distance your reticle moved. Then take that distance, divide it by the number of clicks, 80 in this example, and you have your true movement per click in inches. So if your reticle moved 21 inches you would have a click value of .2625 which is very close to true MOA. if it moved 20 inches it would be IPHY or .25 inch per click. If it moved 22.5 inches your click value would be .281.

    If you want to shoot instead of measuring you simply shoot a base group then dial 20 MOA then measure the distance between the two groups, then divide that number by the number of clicks.

    With some ballistic programs you can actually enter in the true correction per click that you just calculated get the program to read correctly, ie enter .262 or .25 or .281. Other programs want you to come up with a correction factor which requires you to find the error % and then enter that, ie .262 (true MOA) divided by .281(the actual click value you determined) = .929. Then your data card will show correct drops assuming other data is accurate.

    Clear as mud? :D

    Scot E.
     

  3. joseph

    joseph Well-Known Member

    Messages:
    1,238
    Joined:
    Jul 26, 2009
    When I was figuring my drops for my custom turret on JBM and got the drop measurements. Then went to the range to actually shot and measured "MY" rifles drops with a 200 yd. zero and found that "MY" rifle actually dropped more than the JBM program at 200 & 400 yards. I gave the actual measured drops to Vortex so they could match the arc of my bullets path to the custom turret.

    This worked more accurate than the JBM program because when I shot at all 100 yard distances out to 900 yds and the elevation was spot on. :):):):)

    joseph
     
  4. azmetalman

    azmetalman Well-Known Member

    Messages:
    48
    Joined:
    Apr 23, 2012
    Hey guys thanks for the great information. This saved me from a Maalox moment.....Really.
     
  5. Greyfox

    Greyfox Well-Known Member

    Messages:
    3,711
    Joined:
    Jan 21, 2008
    I had an error in my original calculation. At 200 yards 44 clicks resulted in 23.5" which calculates to .267" per click. This translates to a +.5 MOA error with my load that results in the bullet impacting high. I don't have the option in my G7 Br2 rangefinder of adjusting the internal calculator, but can achieve the correction by adding 20 FPS to my velocity input. I will verify this at the range to get actual impact points when I can get to the range. Thanks for the feedback!