The Firing Line Forums - View Single Post - Standard Deviation

saands · September 25, 2000, 11:10 PM

Well ... you got a smile out of me ... Great to see that you are finding the statistics useful. Let me take a stab at the questions ... but I will state up front that I'm better with statistics than I am with reloading. First real question : 454 doesn't reflect the greater s.d. with a larger spread? Well ... I don't know how steadily you hold your .454, but I bet it isn't as steady as your .220 Swift! The contribution of velocity to group size due to trajectory is probably pretty small, so whatever is causing the big groups (shooter, defect in barrel, heating of barrel, lock-up of the action, etc.) is probably more important than the velocity. I'm assuming that the distance is much shorter (.95" at 100 yds would be truly impressive for a pistol

) so the velocity component might truly be tiny not just small.
What if you put the chrono at 100 yds? I would place my money on larger standard deviations and their associated wider ranges in velocities because every bullet is just ever so slightly different and all those differences can't help but add up to a velocity change as they go downrange. A slight burr on one will bleed energy faster, etc. The farther it flies before the measurement is taken, the larger these little effects will become. The effects might be small if the component quality is high, but the ranges should increase. This might be a way to increase your sensitivity if you are trying to rank bullet manufacturers? You will also see some changes from group to group anyway, but assuming you loaded them all the same, the averages for groups of 10 shots should fall in a range of:
R = (6 * s.d.)/3.16
3.16 is the square root of 10 (the number of observations in the group). If an average falls out of that range then something changed!
Enough statistical nonsense ...
Have fun ... I think I might need to get myself one of those chronos. Do they work indoors? ... do indoor ranges let you use them?
Take care,
Bill

[This message has been edited by saands (edited September 26, 2000).]

September 25, 2000, 11:10 PM	#5
saands Senior Member Join Date: November 14, 1999 Posts: 1,573	Well ... you got a smile out of me ... Great to see that you are finding the statistics useful. Let me take a stab at the questions ... but I will state up front that I'm better with statistics than I am with reloading. First real question : 454 doesn't reflect the greater s.d. with a larger spread? Well ... I don't know how steadily you hold your .454, but I bet it isn't as steady as your .220 Swift! The contribution of velocity to group size due to trajectory is probably pretty small, so whatever is causing the big groups (shooter, defect in barrel, heating of barrel, lock-up of the action, etc.) is probably more important than the velocity. I'm assuming that the distance is much shorter (.95" at 100 yds would be truly impressive for a pistol ) so the velocity component might truly be tiny not just small. What if you put the chrono at 100 yds? I would place my money on larger standard deviations and their associated wider ranges in velocities because every bullet is just ever so slightly different and all those differences can't help but add up to a velocity change as they go downrange. A slight burr on one will bleed energy faster, etc. The farther it flies before the measurement is taken, the larger these little effects will become. The effects might be small if the component quality is high, but the ranges should increase. This might be a way to increase your sensitivity if you are trying to rank bullet manufacturers? You will also see some changes from group to group anyway, but assuming you loaded them all the same, the averages for groups of 10 shots should fall in a range of: R = (6 * s.d.)/3.16 3.16 is the square root of 10 (the number of observations in the group). If an average falls out of that range then something changed! Enough statistical nonsense ... Have fun ... I think I might need to get myself one of those chronos. Do they work indoors? ... do indoor ranges let you use them? Take care, Bill [This message has been edited by saands (edited September 26, 2000).]