Ballistic Coefficient

I haven’t post anything in a while.  Between Christmas break and dealing with trying to figure out the pending situation of my reassignment (it’s been a mess) I’ve been mentally preoccupied and doing minimal writing as of late.  That’s not to say there hasn’t been anything to write about, given the situation in France with Charlie Hebdo being brutally attacked, the recent national release of “American Sniper” (awesome movie, by the way), and the subsequent celebrity bashing of said movie.  Frankly, I’ve had my fill of that negativity, however, and would prefer to discuss something else I have been delving back into the last couple days.

I’ve been compiling various manuals on the art of sniping to find different ways that are done by different people.  Each different branch has its quirks and there is much to learn in the details of reading up on what someone else is doing that might improve your performance.  This academic pursuit has there-in led me to dig into my own personal weapons systems and to improve my data.  I’ve been sitting on a Knight’s Armament ballistic calculator for a few years now and never really appropriately used it, not that I couldn’t, I just never needed to.  The simple ballistics of the preprogrammed rifles is as easy as range your target, scroll down, and make it happen on your scope.  However, with my personal rifles they are completely different, with completely different numbers that effect the trajectory of the bullet. One major number, which I am now finding that plays a larger role than I was even led to believe in sniper school, was ballistic coefficient.

Ballistic coefficient (BC), as far as we learned at the school house, was just some amazing number that magically pertained to our round, that was just given to us to punch into the calculator and rarely mentioned.  Upon doing my own research to figure out what number I needed to punch into my calculator for a 250 gr, .338 Lapua Magnum, I actually started reading up on what it means, and how it pertains to making that round hit what I want it to hit.

BC is the round’s ability to overcome air resistance in flight.  It is placed on a scale from 1.000 to 0.000 with the higher number meaning less resistance.  Less resistance from the air means there is less drag to slow the bullet from it’s initial velocity.  For example, M118LR (the standard long range 7.62 round) has an average of 2600 feet per second out of the muzzle.  However, by about 780 meters, the velocity has slowed to 1100 feet per second, thus breaking from supersonic into subsonic.  This loss of velocity at that range, gives M118LR a G1 BC of .495.

Now, I just used a new term there, “G1”.  G1 is the standard of which the scale of 1.000 to 0.000 is measured. It is the common measure, similarly to using metric over standard (which would be the G7 scale).  The G1 scale was made for a round that resembles more of a pistol round with a lot of drag, which is why a round such as M118LR has an average coefficient.  g1g7shapeNow, there are fancy formulas out there that connect the mass, diameter and drag to figure out what your BC is.  Hopefully, and I ended up being lucky in this case, you wind up getting a round whose manufacturer includes the BC.  Is it always accurate?  Not necessarily, but you can use it as a base and adjust your data from there.  A good starting point for your round will be close enough to get you on target, if you find that you are consistently hitting lower than what your ballistic calculator tells you, adjust your coefficient lower so that the given chart matches your impacts.  Inversely, adjust your number higher if you are consistently hitting high.
800px-Effect_of_BC_on_Energy_RetainedAs you can see here, a higher coefficient retains its energy and velocity over a longer distance, as the drag effects it less.
800px-Effect_of_BC_on_Wind_DriftBC effects more than just drag as well.  As this chart shows, a lower coefficient bullet will drift sideways from the wind more than a higher coefficient bullet will.  This, in part, is because of the retained velocity of the round.  The same amount of push is being placed by the wind, but a higher BC is traveling faster and has less time of flight than a lower BC.  Meaning that ultimately, the wind has less time to push the round as far to the side.

To sum all of this data up, a higher BC boils down to having a more precise shot at longer distance.  Compared in this final chart against the .300 win mag, another popular distance round, and given the knowledge that M118LR drops to subsonic at approximately 780 meters, you can see here that with a higher coefficient, these rounds will continue to maintain velocity and energy at a greater range (not even on this 1000 meter chart).

338-lapua-mag-vs-300-win-mag-velocity

A simply explained and easily over-looked factor in making those accurate long shots.  Things to consider when selecting your next caliber/round.

Stay hidden; stay safe.

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