Calculating ballistic coefficient for unknown bullets

[Naterater]

Is there was any way to calculate the ballistic coefficient of a bullet that doesn't have any published data? I have searched online and everywhere for information on some military bullets and found nothing.

Are there any programs or equations that could calculate the general BC? (I'm doubtful on the equation one, but I thought I would ask.) I have 1 chronograph that I can set up for velocity readings.

I was hoping to use this info on milsurp bullets and possibly tracer rounds in .308win for long range, possibly extending past 800 yards.

 

[Zak Smith]

If you have one chrono, a good zero, and know all the environmental and gun setup parameters, you can reverse calculate it from the drop at long range.

 

[Naterater]

and how would this be possible? Would I have to guess and check with ballistics programs or are there any programs that are specifically built for this?

 

[Zak Smith]

Some do it automatically, but if you have the known data per my first post, just start changing the BC value you give to the program until it matches your data.

 

[Sunray]

Hi. The BC of a bullet is a measure of a bodies’ ability to overcome air resistance in flight. Doesn't have anything to do with external ballistics other than a bullet's potential.
Go here. http://gunczar.com/gg.html
If you want to use a .308 past or out to 800, use a 175 grain match bullet. Milsurp bullets aren't made for great accuracy. Certainly not a trace. Friggin' things burn out too fast. Trace isn't made for great accuracy at all.

 

[Zak Smith]

Hi. The BC of a bullet is a measure of a bodies’ ability to overcome air resistance in flight. Doesn't have anything to do with external ballistics other than a bullet's potential.

First statement correct; it would be hard for the second to be more wrong. Other than environmental factors (and gun setup), MV and BC together determine the trajectory (that is the definition of external ballistics).

There's nothing wrong with trying to use milsurp bullets for mid and long range shooting. They probably won't win many matches, but you can definitely make hits. When I was "shooting in" the Steel Safari course, I shot Australian surplus ball ammo in my .308 (so as not to waste my "good" ammo) and I was able to hit targets as far as 1000 yards. FWIW, I just guessed at the BC and it was close enough to be on to about 700 (I guessed 0.4), at 800+ I was a bit off.

 

[Zak Smith]

I would also add that it's pretty common for long-range shooters to reverse engineer an "actual" BC value for the bullet they use, especially when it is thought that the published version is "optimistic." For example, Lapua used to publish 0.615 for the 139gr Scenar (6.5mm) and 0.509 for the 155gr (.30). I know numerous shooters (including myself) who were using values less than these to make the trajectory match at longer distances. We basically would carefully collect a bunch of point of impact data and then run the numbers while modifying the BC until everything matched up. Then ballistic expert Bryan Litz recalculated them from experimental data and published the values in his book, and they were very close to the values that we had been using from our field shooting data.

 

[Naterater]

Thanks guys. I'm sorta using the bullets as tactical plinking rounds. They're dirt cheap and look like a bit of fun. I am looking to train on ballistically dialing in on targets.

Of course for competition or hunting I would use bullets better suited to their uses.

Thanks for the great advice!

 

[gamestalker]

Google " calculating ballistic coefficience" and you should see a site that has a bunch of calculating devices online for free.
If I recall there was also some other calculations like foot pounds of energy.

 

[45crittergitter]

What you should do is clock them at the muzzle and again downrange, then calculate BC using the now-known velocity differential and range. Much more accurate than using drop or some guess based on apparent shape. If your loads are pretty consistent, you should be able to do this with one chrono on the same day by just averaging velocities at each distance.