have some questions.....

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by dogdinger, Aug 17, 2008.

  1. dogdinger

    dogdinger Writers Guild

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    my son and i did some shooting today and i have some confusing data. i punched in all the numbers for the gun and load into the JBM program and they are not real close. its a 300 RUM shooting 180 AB's @ 3250 fps. sighted in to be 1.5 inches high @ 100. JBM says 27.1" of drop @ 500 yds. actual drop is more like 24". OK i can live with that, but when i dial for 24" (actually dialed 20 cliks) im still 4 " high.....my guess is that the scope adjustments are not exact. it is a new bushnell 3200 5.5X15. 16 cliks put us right on target. the scope seems to adjust correctly @ 100yds. 30 cliks put us dead on @ 650yds.......46 cliks @800......it just doesnt work out using MOA either. am i right to assume it is just the scope or am i missing something? AJ
     
  2. AJ Peacock

    AJ Peacock Well-Known Member

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    No ballistics calculator is going to match your rifle 100%. The chronograph might be a little off, the BC of the bullet is different from each barrel and local environmental conditions have a big impact as well. Really, the best use of the calculator's is to get you close, so you can discover that it's 16 clicks instead of 20 etc.

    As long as the scope is repeatable from one range to the other, I'd be happy with it. You could always check the scope, by shooting a box at 100yds with it ( shoot one in the center,
    then 20 clicks down and 20 right shoot one,
    Then 40 clicks up shoot one,
    Then 40 clicks left shoot one,
    Then 40 clicks down shoot one,
    Then 20 up and 20 right and it should be back on center.
    You should have a square with 2 shots in the center.)

    AJ
     

  3. dogdinger

    dogdinger Writers Guild

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    may try that , but as you say, if it repeats(and it seems to) i think i will stick with it....getting good groups and it seems to come back to zero good.
    i havent found the jbm software to be perfect with any caliber ive shot, but it is real close with my 7rm
     
  4. britz

    britz Well-Known Member

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    First, did you enter in an elevation or the Barametric pressure as they are in your area? It doesn't make a huge difference but I've noticed it helps. Also if your origional zero was off a little like 2 or 2.5 inches high at 100 is going to make a difference.
     
  5. cornstalker

    cornstalker Well-Known Member

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    ditto britz.

    Our thin dry and wonderfully clear Colorado mountain air enhances exterior ballistics dramatically. Most ballistics calculators can be set for this, but I still have found fairly large discrepancies.
     
  6. Eaglet

    Eaglet Well-Known Member

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    Dogdinger,

    3250 ft/sec. is the factory given velocity for the 300 RUM by factory ammo when using 180 grains. Are you shooting factory or reloads? Have you chronographed the ammo you're shooting? What elevation did you shoot at? I don't think your scope is wrong because the clicks you used do make sence when using much higher muzzle velocities. Inside the dial cover caps for your scope, does it say 1" = 4 clicks or 1 MOA = 4 clicks?
    That's a lot of questions huh... :)
     
  7. royinidaho

    royinidaho Writers Guild

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    And...........an incorrect measurement of line of sight to line of bore (scope height) will make some difference.

    and as Eaglet said, MOA vs IPHY makes a little difference but I'm not that good yet.:(
     
  8. dogdinger

    dogdinger Writers Guild

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    i did enter the elevation......crono'd the load also, scope is moa....the scope height is just a guess but i cant be off by more than a 1/4 inch. i just cant believe the bushnell would be off by 4 cliks @ 500 yds.....??? AJ