Calculating BC

[wyshadow]

For the past several weeks, I've been having a hard time trying to match my measured drops of my .25 Vulcan shooting NSA 33.5 out to 200 yards to Strelok. I would adjust my 10 to 100 yard's drops to match but my 100 to 200 yard's drops would be off. The reverse would happen if I adjusted my 100 to 200 yard's drops. I finally decided NSA's BC was not right for my Vulcan and my area. I decided to setup my chronograph at 50 yards and shot a 20 shot string. I then I did another shot string at the muzzle and used Pyramyd Air's BC calculator. I also used AGN's BC calculator and got a slightly different number. Now my Strelok and my drop measurements match using Pyramyd Air's. I don't know why I haven't done this before with all my projectiles. I tried using my Labrador a few times to get my true BC but every time I used it, I wanted to use the Labrador as the target. Has anyone else done this to get their true BC?

 

[JungleShooter]

I have done what you have — measuring the BC at two different ranges.
And I got some not so nice data showing that my favorite projectiles have a much lower BC than the one published (yeah, yeah, that was to be expected...).


However, I used a ballistic calculator* that:
▪ takes into account the atmospheric conditions (especially the local air pressure — not sea level, unless your city in New Mexico has access to the ocean.... ).
▪ uses a drag model that is close to the type of projectile I was shooting



Strelok Pro offers you two advanced options that can help you get your BC fine-tuned:
▪ You can enter a custom drag model (and your Labradar will give you the necessary data).
▪ You can enter up to 5 different BC values — at 5 different velocities. That is usually sufficient to true your BC to your actual trajectory.


Cheers,

Matthias


*Link for GPC Ballistic calculator:
Platforms:
Android, iPhone, Windows, Mac, and Linux

GPC Ballistics Applications for Windows, MacOS and Linux

A few ballistics applications for Windows, macOS and Linux written in the PureBasic programming language.

gpc.fotosoft.co.uk

 

[Long_Shot]

I've done that @ 100 yards many times recently. Even @ 200 yards before.
It may not be the most accurate way of getting a "true BC number". But close enough, without a time in air meter.
Then there's which drag model......which app. ???
I even considered getting the $1000. FX radar with different yardages and BC numbers on screen. Appears discontinued though.
The best value for me of this technique is comparing the many different slug types I make, against each other.
At 100 yards the ES should be small, or the round is probably unstable.
Just my .02 ..........