Determining BC

[Mutt]

I trying to get a BC for some 7.62 Nato ammo I have. It's South African surplus ammo and it is 147 gr. Other than that, I know nothing about it. I would like to work up a range card for this ammo since I have 500 rounds of it. Unfortunately, all the ballistic programs need a BC to draw up a drop chart.

My question is ..... how do you figure out a BC for an unknown bullet???

[Drifter]

Chrono to get velocity, then shoot downrange to get actual drop at a known distance based on a shorter range sight in distance. Play with the BC in a ballistic program until it matches your test, then you should have a reasonably accurate drop chart for other distances.

Maybe sight in at 100 or 200 yards, then check drop at 600.


ETA- Winchester USA 147gr FMJ has a BC of .421 according to the JBM Ballistics website, so that might similar to your bullet.

[stanc]

I'd think the BC would about the same as for other FMJ bullets of that weight. A couple years ago, Winchester's site showed a 0.418 BC for their 147gr FMJ.

If you check other manufacturers' sites for 147-150gr FMJ, I'm pretty sure you'll find they are in the 0.40-0.42 range.

[Mutt]

That's helps alot ..... thanks.

[stokesrj]

You will be more accurate in your calculations than that ammo is capable of anyway using a generic BC of .42. Ball ammunition is generally capable of 2 MOA with a rifle capable of .5 MOA with match ammunition.
Bob

[Rambozo]

I believe the BC is in the area of around .390 for M80 ball, and if I remember correctly, my 18" FAL got it going to around 2750fps.

As for determining BC, the JBM website has great tools for that. You can pull a few and measure them. Put the data through the calculator on the website, and out comes a nice BC table.
http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

[independentrider1]

http://cartridgecollectors.org/headstampcodes.htm