Cant get ballistic calculators to line up with actual drops



  • I started practicing shooting 100 to 1200 yards with my 308 that I will be using in a couple competitions in April. Actual drop values were as follows, zero 100yds, 1moa @ 200yds, 14 3/4moa @ 620yds, 56 1/2moa at 1270yds. I am shooting 168gr Berger VLD hunting bullets with an advertised BC of .498 G1 & .255 G7. My chronograph malfunctioned half way through my shooting session so I wasn't able to get a velocity on the bullet, but last time I chronographed this gun was with 168 Nosler BT's and the same powder charge, at 2780fps. When trying to get any ballistic calculator to line up even close to my actual drops, I have to adjust the G7 BC to .195, and drop my velocity to 2760fps. Even then I'm off at 1270yds by about 2moa. Does anything stand out here that I may be missing? I'm going to reverify my data next weekend, but in the mean time I'm just trying to figure out the calculators. For what its worth, my wind calls were pretty much spot on which I think is weird if the elevation differs so much?!?



  • I could just kick myself in the nuts, I didn't even think about Coriolis. How much would Coriolis effect drop at 1270yds shooting south vs due west? And west vs east for that matter?



  • Running a different bullet can/will produce wildly different velocities. Just because they are the same weight, doesn't mean they are the same bearing surface, jacket lubricity, or core composition... among other variables.

    You MUST, and I repeat MUST have accurate velocity data to feed a ballistic computer if you want to have a competent firing solution. Of course there are ways to cheat the system and attempt to "proof" the firing solution in a different way... but that is not the same as proofing a firing solution that will demonstrate the software is properly tracking as the conditions change.

    Having said that, 2moa at 1270 is inside the window of other variables. I suspect there are other variables at play. Was there a crosswind during live fire? If so, how much and from which direction?



  • @orkan yes there were swirling winds, but when wind was consistent and we were getting consistent hits, we were measuring 5 mph wind at 90 deg. Does crosswind effect drop?
    Just wanted to mention, the only way I could fudge the ballistic computers is to lower the G7 BC from .255 to .195. Trying to fudge G1 BC and velocity doesn't come up anywhere close to my 620 & 1270yd actual drop. If I use G7 BC .195 and 2760fps, that's where I'm still 2moa off from actual drop at 1270yds. Trying to fudge the G1 and velocity is 10+ moa off at 1270 while still hitting my 620yd drop.



  • @donnie said in Cant get ballistic calculators to line up with actual drops:

    Does crosswind effect drop?

    Yes.



  • Well I think I'm going to start from scratch and try again with ballistic calculators when I can verify my velocity. I'll also read up on how crosswind affects drop. Thank you.



  • @donnie Google aerodynamic jump.



  • I may be incorrect in referring to what happens here as aerodynamic jump.

    Our range here gets some horrific winds depending on wind direction. It is amazing how a few compass degrees can change a slight wind to a target breaker.

    The wind will come over the mountain range and accelerate thru a valley as it blows downhill to the point I have had to add up to .5 mil on the 600 yard target. We also get the opposite wind occasionally that requires up to .5 mil decrease. I'm shooting mainly an 80 grain .223 projectile @ ~2795. My shooting partner shoots 168 grain .308 and has experienced the same results (although different adjustments).

    This is the westside of the west Maui mountains, flyers avoid them due to crazy up and down drafts. Mid height of power line pole is ~600 yards.
    X3nOVpN.jpg



  • @mamalukino Aerodynamic jump does not have to do with upward and downward drafting winds.

    It has to do with bullets fired in a right twist barrel in a left to right wind causing increased drop and right to left wind causing reduced drop. This is reversed on a left twist barrel. The stronger the wind, the more dramatic the effect.



  • @orkan
    Thanks Greg, I thought vertical winds were in that category.



  • Valleys will likely give you up and down drafts that must be treated as a vertical "crosswind".
    Aero jump is function of gyroscopic motion of spin stabilized projectile with Cp ahead of Cg.
    Good rough rule of thumb is 1/4 minute per 5-6 mph x-wind. In those valleys I would be looking at vertical wind flow...
    Also do not forget atmospherics...



  • Look up gyroscopic precession, that is the proper name for the dynamic phenomenon that is causing this. Tons of website info, videos, etc.

    If you ever played with a toy gyroscope as a kid, you likely witnessed it. Push on the frame with the gyro spinning, and the reaction of deflection occurs at 90 degrees to that of the force being applied.

    Helicopters depend on this effect for flight control.