Drag Function

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Guest, Nov 14, 2003.

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  1. Guest

    Guest Guest

    Some of the trajectory programs need a drag function (G1-G8,GL,GI)for the calculation. What is this and what would I use for the SST and SMK bullets. Thanks
  2. Brent

    Brent Well-Known Member

    Jun 12, 2001

    Email me if you would.

    Different shaped bullets produce much different rates of velocity errosion due to air drag. Most all bullet manufacturers use the G1 drag curve and their published BC's are based on this drag model. The higher the BC number, the closer it comes to equaling the trajectory of the standard projectile with a 1.0 BC.

    A bullet with a specific "shape" models each drag curve, G1, G5, G7 etc, and each is a 1" diameter, 1 pound projectile. This "standard" projectile as it's called, for each drag curve represents a BC of 1.0 and all others of its same shape are asigned a relative BC number acording to its rate of velocity decay as compared to this standard projectile. Most all small arms projectiles weigh much less than 1 lb and thus the rate of velocity errosion is much faster, so their BC numbers result in a number smaller than 1.0.

    The trajectory curve for a G1 shaped bullet has a higher midpoint in its arc, and simplified quite a bit, it flies more like an arrow than a bullet, because its standard projectile has a nose shape nearly the same as that of a 45 ACP ball round, round and very unaerodynamic to say the least.

    Trajectory tables for todays modern streamlined boattail bullets have been more closely modeled with the use of G5 and G7 shaped standard projectiles.

    You will notice a bullet flies flatter with the use of the G5 drag function, and even flatter yet with the G7 curve, as their standard projectiles have less air drag, and as a result these will more accurately model your actual bullet flight and drop at all ranges, however, you can not enter in the same BC number you most likely have seen advertised for the bullet you've selected to use. The MFG's use BC numbers based on the G1 curve, and as a result they produce much higher BC numbers than they would if compared to the standard projectile of their own shape.

    Some ballistics programs will convert the G1 BC to any of the other drag curves BC so you can compare their predicted drops at all ranges to those that you've established from actual drop data. I have the Oehler Ballistic Explorer and RSI Shooting Lab programs that will perform this conversion.

    Knowing that MV and BC are the two functions that determine any bullets trajectory, and you know the MV and drop for your load at say 600 yards, you can enter the MV in and adjust the BC until the predicted drop matches the actual drop you've noted at 600 yards. The BC that does this is good for that drag function in those atmospheric conditions the drop data was collected in. The muzzle velocity must be accurate if you determine the bullets BC this way.

    Hope that helps some, and wasn't too confusing [​IMG]
  3. Guest

    Guest Guest

    Thanks Brent. I think I get the picture. If I use the advertised BC and the G1 drag I should be close right? Than do the range work to get it right on the money. If i'm wrong let me know and i'll try again. Thanks again