Drop Charts- Please can you help me ???

Discussion in 'Long Range Hunting & Shooting' started by robbobsam, Aug 9, 2010.

  1. robbobsam

    robbobsam Well-Known Member

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    I have just been trying to make a ballistics drop chart for my rifle.

    Its a .243 Tikka Super Varmint.

    I am shooting Norma factory ammo and the box says the muzzle velocity is 1035m/s which equates to 3395ft/s. The ammo is 75grain hornady v-max heads which have boat tailed bullets and the ballistic coefficient for these bullets is .33.

    The scope is mounted 1.5" above the barrel and I zero the gun at 100yds.

    I have tried the Burger Bullet software that you have pinned on here and I have compared it to the other software that I have used which is here>>>Other software

    What I find confusing is that they give massively differing figures [​IMG] [​IMG] [​IMG]

    Can any of you guys enlighten me ???
     
  2. kiwi3006

    kiwi3006 Well-Known Member

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    I haven't had a chance to check both calculators, but something to check is the altitude, pressure and temperature that the programs are using.

    I used a program called Bal Cal. Using your data and std atmosphere at 1000 ft asl the drop at 600 yds is 66.9" or 11.1 MOA. How does that compare?

    Stu
     

  3. hey_poolboy

    hey_poolboy Well-Known Member

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    I regularly use this site and they have proven to be pretty accurate with my hand-loads as long as you can locate the correct BC.
     
  4. robbobsam

    robbobsam Well-Known Member

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    The Big Game balistics calculator gives me 66.9 inches or 43 clicks which equals 10.3 MOA

    The berger bullet site gives me something completely different, it gives 7.88MOA or 49.51 inches.

    The berger bullet software gives you a choice between g1 or g7 and I belive that differentiates between flat and boat tail bullets. Well my 75grain v-max are boat tailed.

    The annoying thing is the software indicates that the figures generated for the flat bottomed bullets is closer than what it generates for boat tailed.

    Whats happening.
     
  5. kiwi3006

    kiwi3006 Well-Known Member

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    G1 and G7 are quite different numbers the .33 BC is G1 BC.

    For example the 168 gr .284 berger has a G1BC of 0.617 but a G7 BC of 0.316.

    Unless it says that it is G7 BC assume that it is G1. Berger is about the only company that publishes G7 BC.

    You are right that the G1 BC is based on a flat based bullet and G7 is a boat tailed bullet. The G7 BC will give better results for a boat tailed bullet, but you need to make sure that the BC you are using is a G7 BC.

    Stu
     
  6. robbobsam

    robbobsam Well-Known Member

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    Thanks Stu,

    I will try that now.

    How accurate are the stated velocities on the side of the Norma Bullet Boxes ???

    Will I be better off just getting a chrono.
     
  7. kiwi3006

    kiwi3006 Well-Known Member

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    I would get a chrono. I have never chronoed a norma round so can't help there.

    But you should validate your drop chart as of course. ie shoot at 300 yds (see comment below). Compare where it is hitting compared with the drop chart. do the same for 600 yds and further is possible. Then tweak the velocity or BC until the drop chart agrees with your actual results.

    This assumes you know the barometric pressure and temp and humdity when you were shooting.

    Re 300 yds. I zero at 100 yds. Then I dial in my 300 yd comeup and shoot at 300 yds and adjust turrets so that I am zeroed at 300 yds. I then "zero" my turret so that it reads what the 300 yd comeup states. ie for my load my 300 yd come up is 2.9 MOA. I zero at 300 yds then zero the turret to read 2.9 (well actually 3 as it is a 1/4 MOA turret.

    Stu.
     
  8. royinidaho

    royinidaho Writers Guild

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    Something is awry when two ballistic codes don't closely agree.

    Maybe its because you guys are upside down down there and the bullets are falling up instead of down.:D

    One to try is JBM ballistics. Google will find it quickly.

    What you may try is to shoot at a very large target at several distances out to your max distance. This will tell you what your drops are. The two variables you won't be sure of are MV and BC. But you will know your drops and hitting stuff is what's important.

    0.33 is a quite puny bc for a 243 projectile. I haven't looked it up but would be much more comfortable with a 0.6 or a little better.

    Just a thought....
     
  9. robbobsam

    robbobsam Well-Known Member

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    Guys,

    Thanks for that. This site is a great source of info for novices like me.

    Thanks again.