Ballistic Coefficient (BC) Testing and Analysis

DallanC · Site Admin Joined: Jan 18, 2005 Posts: 3572 Location: Utah

I find it a little wierd on a website created for a specific ballistics software, to see links to competitors software. Ah well I guess.

Anyway Sierra feels BC should be bracketed depending on velocity. For a specific bullet there are usually 3 BC values depending on velocity. I've had more than a few requests to update PointBlank to support this... but as they are the only MFG to my knowledge using multiple BCs, I've never found it worth the effort.

Also, why does no-one talk about bullet deformation due to different rifling configurations? A BC is only good for a specific barrel. Riflings cut deeper or shallower will deform a bullet in different ways, affecting the overall shape of the bullet, which in turn affects BC.

Ever looked at the bore of a Newton rifle?

-DallanC

PaulS

The published ballistic coefficients are corrected for altitude, barometric pressure and zero humidity. If they weren't then the BC would be meaningless. Using a G1 BC in Point Blank and other good software adjust the "working BC" for the altitude and other parameters that exist when and where you shoot.
You got a BC calculated at 1800 feet with 29.40 pressure and 45% humidity then it would have to be lowered at sea level at 29.91 pressure and 60% humidity to match the trajectory there. If I used your calculated BC (while assuming it was corrected to sea level standard atmosphere using Point Blank at 2000 feet above sea level and 80F with 10% humidity the software would come up with nonsense because the software would reduce the friction for my 2000 feet and higher temperature while increasing the drag for the low humidity.
Dallan shows reason for more deviations in BC and how about the tipped bullets that overheat and melt away in flight? That will give a lower BC than if you fired the same bullet at a lower velocity even under the same environmental conditions.
The BC provided by a manufacturer has to be corrected to the standard atmospheric conditions to be meaningful when used in software under different conditions or you get garbage out. The software you use might make those conversions but it might not too. Before using measured or calculated BC for reference there must be a reference point. The standard atmosphere is the "normal" reference point for BCs.

Elvis

shooting a 130-180grn projectile at 2900-3000fps at 0-350 yards
you sight in to be 3" high at 100 yards and forget all the other B.S. and just go shoot you some deer.
yip the flat or round nose jobbie wont hold up as good as the sleak splitzer with superduper arodynanically designed nose cone and moly coating to make it slipperyer than a whores drawers
BUT crunch the numbers on my 170grn RNFB speers going say 2800fps from my .270 and you will see what I mean..at the sub 300 yard range where lets face it 90% of animals are shot,the difference is sweet stuff all.

slimjim

DallanC wrote:

I find it a little wierd on a website created for a specific ballistics software, to see links to competitors software.

Dallan, my apologies. The computer I use cannot have any software apps loaded on it that are not approved. Thus I use on-line ballistic calculators. If you have any suggestions to reference Pointblank, I'm open. I just doing have any experience with it.

slimjim

PaulS wrote:

The published ballistic coefficients are corrected for altitude, barometric pressure and zero humidity. If they weren't then the BC would be meaningless.

PaulS, I thought I explained that the BC derived from this testing were corrected for altitude, barometric pressure and humidity. One also has to correct for temperature and winds. You are correct, in the situation where I didn't have accurate altitude, barometric pressure, and humidity, the BCs were meaningless.

slimjim

Elvis wrote:

BUT crunch the numbers on my 170grn RNFB speers going say 2800fps from my .270

Elvis, the 150s I shoot out of my .270 are pretty stout at 2800 fps. What powder are you using to get a 170gr to 2800 fps?

Elvis

pushing them with ar3312 and a couple of grains over book max 54grns..... as for how fast they going........ dont own a chrony.

Elvis

nick harvey has in text 56grns of 4831 for 2800fps...he had longer barrel and those early loads of his are hot.

PaulS

Slimjim,
You may have said it and I just missed it or forgot seeing it. Either way the BC has meaning under those conditions for you. If the software does it all and gives you the corrected number then it is golden!

slimjim

PaulS, with my test results consistent with Applied Ballistic and repeatable, it gives me confidence the process is working.

hunterjoe21

PaulS

I am using a LabRadar chronograph and it provides velocity at every pulse so I have the information I need to calculate the BC of any bullet I use. With all the complementary information provided I am surprised that the firmware doesn't calculate the BC for me. Smile

I have a number of ballistic programs and the two I use most often are Point Blank and my own that I wrote for DOS many years ago. The traces from te two pieces of software are so close out to 300 yards that for all practical purposes they are the same. My software is less accurate with pistol bullets than Point Blank so when I want to predict path for my pistol rounds I always use Point Blank - even though I have to use my wife's computer to do it- hers is the only one that runs Windows OS.

slimjim

PaulS wrote:

I am surprised that the firmware doesn't calculate the BC for me.

To get an accurate BC, the unit would also have to measure all the environmentals, e.g., temperature, humidity, barometric pressure, altitude, and wind.