Finding a bullets BC
- Thread starter Bitteroot
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If the new Garmin calculates BC, I would just get that one. This is great, but I'd like to have one mounted to gun when I'm shooting from the truck seat, or away from the bench. These were 1K and came out right before the first Garmin unit. I think they're about $600 now.
Bitteroot
Well-Known Member
I have the Garmin but only one.
They are very good and very hand although I've not hook it up to my laptop , my buddy has a shotmarker that he sets up at 1 k that I believe gives a target velocity but I don't know how accurate they are plus you need to be inside the sensors.
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lancetkenyon
Well-Known Member
Shoot over a chronograph at various distances, and input actual drops and MVs into a good ballistic calculator.
Bitteroot
Well-Known Member
I like that~ however and if I'm understanding you..
The Chrono needs to be somewhat in line with bullet path, one thought was to shoot through a chrono hoops at the target and that's a thought but quite easy to shoot your own equipment also.
Remember the BC (bullet drag coefficient) formula uses velocity as one of the functions. Velocity changes constantly during flight and is not linear, therefore, the BC number changes constantly over distance traveled. Not linear because, the faster the bullet travels, the more drag is exerted on the bullet which demands a weighted average calculation. Example, there is more bullet drag at 100 yds, than at 500 yds, than at 1000yds. There is not one BC number for a bullet, BC is a weighted average number based on a given distance (decreasing velocity) the bullet travels. The BC for 500 yds is different than the BC for 1000yds for the same bullet. Consequently, to calculate the average BC, you need the muzzle velocity and the velocity of the bullet when it hits the target. The more readings of velocity between these two points, the more accurate your weighted average BC calculation (which is the bullet drag coefficient). Soooo, you need to specify the target distance you want the average BC to represent. Then use that BC number, but don't expect it to be perfect for all distances because it's a compromised number (average number over a distance). A good ballistics program tries to estimate what the average BC (drag) is when you plug different ranges to the target into the calculation and it spits out a come-up number for dialing or holdover. Temperature, humidity, density altitude, etc are also used to calculate the bullet drag. BC is not one number.
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Buy a 30,000.00 Dopler radar system...will tell you right away!!
Bitteroot said:
How does a guy go about finding an unknown bullet BC?
The easiest is to use your ballistics calculator. Shoot a target at various distances, out to at least 500 yds. Don't dial, hold the crosshair on the same bullseye. Then input the drop measurements into the calculator and it will spit out a BC. Make sure you include accurate temp, humidity, density altitude into the calculator. NOTE: this is not the most accurate method due to the error introduced from your rifle's group size, but it will give you a starting point to fine tune from.
How does a guy go about finding an unknown bullet BC?
The easiest is to use your ballistics calculator. Shoot a target at various distances, out to at least 500 yds. Don't dial, hold the crosshair on the same bullseye. Then input the drop measurements into the calculator and it will spit out a BC. Make sure you include accurate temp, humidity, density altitude into the calculator. NOTE: this is not the most accurate method due to the error introduced from your rifle's group size, but it will give you a starting point to fine tune from.
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Bitteroot
Well-Known Member
I
this task is likely far above my pay grade to accomplish but this certainly is the input I've been looking for.
From my country boy position it's probably the best approach considering the time frame I have is to leave the custom bullets out of hunting equation and stick with the Berger's 105 VLD , as likely as it is to have shots from 100 to 865 or 1106 yards.
Now we're into the meat and potato's, thank you !!!
this task is likely far above my pay grade to accomplish but this certainly is the input I've been looking for.
From my country boy position it's probably the best approach considering the time frame I have is to leave the custom bullets out of hunting equation and stick with the Berger's 105 VLD , as likely as it is to have shots from 100 to 865 or 1106 yards.
You don't need to measure MV of each exact shot and match it to that exact drop of that shot. Get an average of a few shots and use that for MV and drops. The more shots, the more confidence in the estimate of MV and drops. You can shoot 5 shots through a chrono, then shoot 5 to measure drops so you don't have to line everything up perfectly. By the way, this is a huge advantage of modern chronos, that you can do both at the same time very easily.
What Lance said is what I was saying but I was a lot less specific, and I probably wasn't specific enough. The main variables you need for a ballistic solution are MV, environmentals, bullet and associated data, BC, rifle twist, distance of scope above barrel, etc - and with those, a ballistic calculator can tell you the drops.
But since you know the drops, or can know them by observation, but don't know the BC, you solve for BC. The easiest way I know to do that is to enter all the variables but BC into a ballistic calculator and plug in different BCs until the predicted drops match the observed drops.
