Defensive Carry banner
1 - 1 of 1 Posts

·
Super Moderator
Joined
·
13,601 Posts
Discussion Starter · #1 ·
Because I've used MagnetoSpeed, which I really like, in conjunction with a ballistics calculator, Strelok ($8) which I really like, I was holding my breath as I compared real world measured data from LabRadar, that boasts a 0.1% accuracy, to theoretical data calculated by a ballistics calculator.

Basically, a ballistics calculator uses a drag model, a bullet's ballistic coefficient, and muzzle velocity to calculate the trajectory of a bullet's flight. Is this useful? You bet it is! That tells us how much bullet drop we will have at any distance!

Typically, we measure the muzzle velocity with a chronograph and accept on faith the manufacturer's ballistic coefficient for their bullet. I trust chronographs. I've seen enough corroboration to believe they give us results with enough accuracy to not be of a concern. But ballistic coefficient from the manufacturer - how do we really know?

Actually, there is a way to determine a bullet's BC with just a chrono, but it is arduous to say the least. You measure the muzzle velocity, zero at 100 yards, shoot at 300 yards, run down to the 300 yard range and measure how much the bullet dropped, feed the muzzle velocity and drop into your ballistics calculator and it calculates your BC. Sounds easy when you say it fast like that! It isn't easy. Wind, your energy level, all kinds of variables can influence the accuracy of the effort.

But now with LabRadar that can measure bullet velocities down range, we can refine the BC a more convenient way. Here's how I did it and I was pretty surprised at what I found.

I chrono'd some IMI 69 gr BTHP MK ammo using LabRadar. I opened the LabRadar report for this series of shots into an Excel spreadsheet and picked one of the shots to compare with Strelok's calculated trajectory. So I fed the shot's muzzle velocity into Strelok and used Strelok's bullet library to get the BC which was 0.301. I had LabRadar set to 5, 10, 50, 75, and 100 yards, so I took Strelok's calculations at those yards and compared the data, here's what I got:

BC = 0.301
yards05105075100
LabRadar259125782565245623912345
Strelok259125752559244623752305
% diff0%0.116%0.234%0.407%0.669%1.706%

That's pretty close for real-world measured compared to theoretical calculations based on one real muzzle velocity and somebody's BC.

But what if the BC isn't quit right? What if it's a little low? What if the BC were actually 0.34 instead of 0.301?

Well I went into Strelok and changed the BC to 0.34 and got this:

BC = 0.34
yards05105075100
LabRadar259125782565245623912345
Strelok259125772563246223992337
% diff0%0.047%0.090%-.244%-0.335%0.345%

But can we justify making such a big change, some 13%, to the BC? Yes! Here's why: We have real-world, measured velocities. These were taken with a machine that is spec'd at 0.1% accuracy, so we KNOW what the velocity should be at each range. If we give Strelok the muzzle velocity measured by LabRadar, and IF the BC is correct, and IF the algorithm commonly used in ballistic calculators is reasonably accurate, then the ballistic calculator should get pretty close to the measured velocities.

Just using the manufacturer's BC, Strelok calculated velocities that were well under 1% difference! Except for the last data point at 100 yards. I know that 100 yards is pushing the limit for LabRadar on that small of a bullet, so we may have some error for that velocity.

But, when I increased the BC from 0.301 to 0.34 and had Strelok calculate based on that BC, the overall % difference, even including that 100 yard point, dropped from 1.7% to 0.345% - and 0.345%, well under 1%, is the worst case. So by changing the BC, the theoretical calculated came into very close agreement with real-world measured.

I am quite impressed that we can use two, such different approaches, and have them agree to within 0.345% of each other.

The other thing I am quite impressed with is that the manufacturer's BC was off by about 13%.
 
1 - 1 of 1 Posts
Top