Post by DBinNY on Jul 27, 2008 17:21:32 GMT -5
I’ve been playing with the PointBlank softwear that we have a link to in the tips and hints section. I was messing around with the chronograph the other night and had it set up 5 yards in front of the muzzle. I was shooting 63 gr of Reloder 7 and 300 gr XTPs with the harvester short black sabots. The recorded velocities of my shots were 2186, 2243 (this one loaded hard!) and 2228 giving me an average velocity of 2219. Well, that’s 2219 at 5 yards but I wanted to know what the velocity was at the muzzle. This may be a minor concern to some but I suspect that many of the members of this board may want to know that as well.
So… being somewhat mathematically inclined and plenty stubborn, I went home determined to come up with a method of figuring it out (there may be a fast easy way to do this but I’m not aware of it, feel free to let me and others know if you know of one). For those of you unfamiliar with it, PointBlank will calculate, among other things, velocities at certain yardage intervals (10, 25, 50 or 100) when you supply a velocity, ballistic coefficient, bullet weight and diameter. If you plug your chrony readings into the target in PointBlank it will calculate your average velocity from readings taken at what ever distance your chrony is and treat that as if it was the velocity at the muzzle.
I created an x,y scatterplot in MS Excel with the x’s being the true yardages and y’s being the velocities (from PointBlank) at each range. These are the dots on the top right graph. I then added a trendline (the curve connecting the points) that basically builds a regression model for you. I found that a 3rd order polynomial provided a great fit to these data. The resulting equation allows you to predict velocity (y) at range (x). Remember that our main goal here is to predict velocity at the muzzle. You’re really not supposed to use regression to predict values outside the range of values used to build the model but the fit was so good I decided to go ahead and do it anyway.
I built another model using everything except the muzzle velocity for a 130 gr .277 bullet (I also wanted to try something fast and sleek) with 25-yard increments so the distance to the chronograph was 25 yards and the model predicted the muzzle velocity within 0.2 fps. This gave me confidence that I could predict muzzle velocity pretty well with a distance of only 5 yards to the chrony.
Anyway, the model predicted that the velocity at the muzzle was 2240 (blue field in the spreadsheet). I then plugged the estimated muzzle velocity into PointBlank. Column D is the regression predictions and column E is the new numbers from PointBlank using the velocity corrected to the muzzle. Column F is the difference between the predictions and the actual (if we choose to accept the PointBlank data as actual). This model slightly overestimates velocity to about 125 yards then slightly underestimates it thereafter. Note that the maximum error is only 2.5 fps and that occurs at 300 yards.
In a nutshell, this thing is fairly easy to play with and if anyone has MS Excel and wants to play with it PM me your email address and I will send it to you along with some directions. If you don’t have MS Excel, you can send me your bullet weight, diameter (e.g., .452), ballistic coefficient, average velocity and distance to the chronograph and I’ll do it for you. You’ll also need to download the free PointBlank software available at: www.huntingnut.com/index.php?name=News&file=article&sid=3
So… being somewhat mathematically inclined and plenty stubborn, I went home determined to come up with a method of figuring it out (there may be a fast easy way to do this but I’m not aware of it, feel free to let me and others know if you know of one). For those of you unfamiliar with it, PointBlank will calculate, among other things, velocities at certain yardage intervals (10, 25, 50 or 100) when you supply a velocity, ballistic coefficient, bullet weight and diameter. If you plug your chrony readings into the target in PointBlank it will calculate your average velocity from readings taken at what ever distance your chrony is and treat that as if it was the velocity at the muzzle.
I created an x,y scatterplot in MS Excel with the x’s being the true yardages and y’s being the velocities (from PointBlank) at each range. These are the dots on the top right graph. I then added a trendline (the curve connecting the points) that basically builds a regression model for you. I found that a 3rd order polynomial provided a great fit to these data. The resulting equation allows you to predict velocity (y) at range (x). Remember that our main goal here is to predict velocity at the muzzle. You’re really not supposed to use regression to predict values outside the range of values used to build the model but the fit was so good I decided to go ahead and do it anyway.
I built another model using everything except the muzzle velocity for a 130 gr .277 bullet (I also wanted to try something fast and sleek) with 25-yard increments so the distance to the chronograph was 25 yards and the model predicted the muzzle velocity within 0.2 fps. This gave me confidence that I could predict muzzle velocity pretty well with a distance of only 5 yards to the chrony.
Anyway, the model predicted that the velocity at the muzzle was 2240 (blue field in the spreadsheet). I then plugged the estimated muzzle velocity into PointBlank. Column D is the regression predictions and column E is the new numbers from PointBlank using the velocity corrected to the muzzle. Column F is the difference between the predictions and the actual (if we choose to accept the PointBlank data as actual). This model slightly overestimates velocity to about 125 yards then slightly underestimates it thereafter. Note that the maximum error is only 2.5 fps and that occurs at 300 yards.
In a nutshell, this thing is fairly easy to play with and if anyone has MS Excel and wants to play with it PM me your email address and I will send it to you along with some directions. If you don’t have MS Excel, you can send me your bullet weight, diameter (e.g., .452), ballistic coefficient, average velocity and distance to the chronograph and I’ll do it for you. You’ll also need to download the free PointBlank software available at: www.huntingnut.com/index.php?name=News&file=article&sid=3