Legionnaire
Contributing Member
If you're using a ballistics calculator or app to calculate firing solutions or trajectory tables, you need the bullet's diameter (caliber), weight, and ballistic coefficient (BC). What if you don't know the BC?
I found myself in that situation. I have a bunch (and I mean a bunch) of old Remington .243 caliber 80 grain PowerLokt HP match bullets. They have been out of production for a long time and I couldn't find a ballistic coefficient listed for them anywhere. Then it occurred to me: my LabRadar captures a projectile's velocity at up to five distances beyond the muzzle; I should be able to use those data to calculate (or at least estimate) the ballistic coefficient.
I already had some .243 AI case-forming loads built with these bullets, so a short range trip produced the data I needed. I then sat down with JBM Ballistics' trajectory calculator. I plugged in all the data I could to get as accurate an estimation as possible. That included the info I had about the bullet, as well elevation, air pressure, humidity, etc. I set the calculator's range increments to match the reading distances in the LabRadar data--in this case 15 yard increments (15, 30, 45, 60, 75). I found the BC for another 80 grain bullet to use as a starting point. Then entered my measured muzzle velocity and hit "calculate."
The resulting table showed the projected velocities at each of my measured distances. They were slightly higher than the velocities measured by the LabRadar, indicating that the test BC was too high. So I played with the BC number until the projected velocities matched the measured velocities. I used the data from several additional shots to confirm. So now I have a pretty good BC to use in my ballistics app. I should have opportunity to shoot at 500 yards on Saturday, which will let me confirm.
I really like the LabRadar.
I found myself in that situation. I have a bunch (and I mean a bunch) of old Remington .243 caliber 80 grain PowerLokt HP match bullets. They have been out of production for a long time and I couldn't find a ballistic coefficient listed for them anywhere. Then it occurred to me: my LabRadar captures a projectile's velocity at up to five distances beyond the muzzle; I should be able to use those data to calculate (or at least estimate) the ballistic coefficient.
I already had some .243 AI case-forming loads built with these bullets, so a short range trip produced the data I needed. I then sat down with JBM Ballistics' trajectory calculator. I plugged in all the data I could to get as accurate an estimation as possible. That included the info I had about the bullet, as well elevation, air pressure, humidity, etc. I set the calculator's range increments to match the reading distances in the LabRadar data--in this case 15 yard increments (15, 30, 45, 60, 75). I found the BC for another 80 grain bullet to use as a starting point. Then entered my measured muzzle velocity and hit "calculate."
The resulting table showed the projected velocities at each of my measured distances. They were slightly higher than the velocities measured by the LabRadar, indicating that the test BC was too high. So I played with the BC number until the projected velocities matched the measured velocities. I used the data from several additional shots to confirm. So now I have a pretty good BC to use in my ballistics app. I should have opportunity to shoot at 500 yards on Saturday, which will let me confirm.
I really like the LabRadar.