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How do I calculate true muzzle velocity from my chronograph? If the readings are taken 6 to 7 feet from the muzzle how much velocity is lost? I'm trying to calculate drop at long ranges for a hunt I am going on. My range is only 150 yards and I want drop out to at least 400 yards. Not sure how much of a difference it would make but just want to be as accurate as possible with my calculations.
Almost nothing and most drop figures are made from chronograph readings taken at 10'. There is no way to figure exact drop with what you shoot except to shoot the distances yourself and measure. Even where you live and weather changes drop. Book figures are entertainment.
Depends on the velocity and BC. If you have the actual BC, or one very close, there is a formula. Easier is; if the distance from the muzzle to the center of the screens is 11 - 17 ' then the correction for most rifle cartridges will be 10 - 15 fps.
All that is well and fine but the difference will be of little consequence in actuality. More important is to get a correct measurement of the heigth of the sights (center of the scope) above the center of the bore. I suggest you just use the measured average velocity that is taken with a sufficient sample (10 shots minimum). I have been doing that for many years and it works out very well if the heigth of the sights is correct.
Larry Gibson
Too True!Quote:
Originally Posted by 44man
Oh. I was thinking about Playboy. Sorry...
I've always read figure 1fps per foot of distance between muzzle and chronograph.
Having said that, there are far more important variables than 10-12 fps, as some have already pointed out.
I would maintain that the only way to know the EXACT drop would be to shoot it @ 400yards. Also, only then would you know if you really were qualified to make a 400 yard shot on a game animal. I'll readily admit I don't shoot at 400 often at all (once every couple of years) and would be hesitant to attempt it without a VERY (bench) good rest and little to no wind.
Shooting at 400 will give you the confidence to do it, calculating 400 will give you a piece of paper to tape to your stock.
Back in my varmint hunting days I would sight my 220 Swift at 350 yards. The using the same center hold, shoot 400, 450, 500, 550 and 600. Then measure the drop. I would then figure the scope settings and still shoot to get centered.
Those figures were taped to the scope tube.
I used a .222 for up to 350 yards and a .44 BH to 100.
I miss the old days!
Velocity loss of a bullet is a function of Initial Velocity, Distance, Ballistic Coefficient and Atmospheric Conditions. For short distances we can ignore the Atmospheric Conditions.Quote:
Originally Posted by DAFzipper
There is a formula that will closely estimate this loss at normal chronograph distances:
You will need a scientific calculator to do the calculation. If you don't have one. Window's calculator will work. Click Start ~ All Programs ~ Accessories ~ Calculator to bring it up. Then click the "View" menu and select "Scientific".
On a scientific calculator, the "e" function is the inversion of "ln" (log number) function and that is activated by pushing "shift" or "inv" + the "ln" key.
Now:
Let CD = The Distance, in feet, from the muzzle to the center of the chronograph screens.
Let BC = The Ballistic Coefficient of the bullet.
Let CV = The average of the shot chronograph values.
Let MV = The Muzzle Velocity calculated from the Chronograph Velocity.
Then
e( CD / BC / 8816) x CV = MV
Here's how to do it.
CD = 7 feet.
BC = 0.300
CV = 1500 fps
On your calculator, enter the following, being sure to enter the parentheses "( )" characters in the equation. The (*) may also be entered as (X) on a hand held calculator and the "=" may be entered as "EXE" or "Calc"
"shift" + "ln" ( 7 / 0.3 / 8816 ) * 1500 =
This should return an answer of 1503.975 fps
Let's do my 280 Remington shooting a Hornady 139 gr. bullet with average chronograph values of 2818 fps
Chronograph distance is 15 feet.
Ballistic Coefficient of the Hornady Bullet is 0.392
e( 15 / 0.392 / 8816 ) x 2818 =
This returns a Muzzle Velocity of 2830 fps
Hope this helps. If a procedure is not clear enough, I can try to elaborate.
Tom's example picked up 12 fps.
Heck, some of my most accurate reloads have extreme spreads that are three or four times that amount.
Don't worry too much about calculations and the numbers; find a place to do some long range practice.
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To get an accurate muzzle velocity either user a ballistic calculator program like the JBM ballistics website or the Aardvark Muzzle Velocity app.