Determining the BC of a slug

[Corvid_hunter]

I bought a GMI swaging die to make slugs and was wondering if there was a way to determine the BC of a slug. I have Strelok Pro and used the Nielsen 34.9gr slug with a BC of 0.11 for my 35gr slug and it seems to be fairly close, but it would be nice to know the actual BC.

 

[Kgphotos]

You can determine the BC of any projectile with a chronograph. Two chronographs work even better but I always use just one. Here is how I do it. Shoot at least ten shots( the more the better) to get the muzzle velocity. Then move your chronograph out to 25 yards and do it again. Then repeat the process again at 50 yards. Now you have the average velocity at the muzzle and two known distances down range. Using a ballistic app you can determine the BC by the velocity lost at each known distance. I use ChairGun but I imagine that Strelok Pro will also work. You will find that BC is a variable that is always changing. The temperature, humidity, altitude, velocity etc all have an influence. The BC you get one day will most likely be slightly different the next day. The method I described will work very well for creating a drop chart that will work for most situations. Large changes in altitude will require a new drop chart in my experience.

Kenny

 

[Corvid_hunter]

Thanks for the information Kenny.

 

[M0ist0ne]

Just make sure you don't shoot your chrony!

Until I can buy a labradar, that's the method I use. 

 

[TrinityMaker]

I've heard the best way to determine BC is to set up a chrony at the muzzle, and set up another downrange in front of the target (say at 50 yards). Then you have a velocity for each round at the muzzle as well as at the target for each individual shot. If you do this for 10 shots, you can add up the numbers and divide by 10 to get the average BC. Most shots will vary in velocity, so this way you can track the BC for each individual slug at it's muzzle velocity to attain an average over the 10 shot group. But you're running 2 chronies at the same time.

 

[Ydunn.223]

What you need is speed at muzzle and the speed at 50 yards

and use a good ballistic calculator like this one from JBM , just fill in 

the information and I like the RG4 profile for airgun use 

http://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi

 

[TrinityMaker]

Not sure I agree with the simplicity of this method. You need to know both velocities in order to calculate BC. You can guess, but it's not going to be as accurate as taking the time to get the different velocities at muzzle and target, then plugging in those to get the BC. 

 

[qball]

All the NSA slugs I have used the BC numbers have been spot on! As far as my limited shooting with the 34.9 grain goes the BC is dead nuts on! 

 

[bigHUN]

Why you cannot calculate from a POI drop?

 

[Corvid_hunter]

With the approx BC I'm using (942fps) a 30 yard zero would have a 1" drop at 60 yards. 100 yards would be 9". 2 chronies might be easier.