Pellet weight and standard deviation

[PlanB]

Using 2 identical pcp air rifles except different calipers, would the larger caliber have a  lower or better S D number?
Thanks

[grauhanen]

SD results are independent of the caliber of the gun.

[PlanB]

I thought that it would take a smaller psi to move a 177 than it would to move  25 so a psi difference would affect the lighter pellet fps more than it would the heavier.
I have been known to be overcome by my brilliance however. I may have overthunk this one!!

Thanks for reply

[Bill G]

I thought that it would take a smaller psi to move a 177 than it would to move  25 so a psi difference would affect the lighter pellet fps more than it would the heavier.
I have been known to be overcome by my brilliance however. I may have overthunk this one!!

Thanks for reply

It would be easy enough to consider that as conventional logic, but lets have a look.   .177 pellet has a cross sectional area of  .025sqr in and .250 has an area of .049sqr in.  If we apply 1800psi to both of these cross sections we get applied forces 45lbs and 88lbs.  the mass that has to be moved and frcition will have a larger influence than the psi.  if the pellets weighed the same, the .25 would go a lot faster.  to over simplify, you are moving .0014lbs(10gr) using 45lbs of force(.177) and .0035lbs(25gr) with 88lbs of force.  Friction isn't considered at this point. 

Hope that helps
Bill

[grauhanen]

I thought that it would take a smaller psi to move a 177 than it would to move  25 so a psi difference would affect the lighter pellet fps more than it would the heavier.
I have been known to be overcome by my brilliance however. I may have overthunk this one!!

Thanks for reply

It would be easy enough to consider that as conventional logic, but lets have a look.   .177 pellet has a cross sectional area of  .025sqr in and .250 has an area of .049sqr in.  If we apply 1800psi to both of these cross sections we get applied forces 45lbs and 88lbs.  the mass that has to be moved and frcition will have a larger influence than the psi.  if the pellets weighed the same, the .25 would go a lot faster.  to over simplify, you are moving .0014lbs(10gr) using 45lbs of force(.177) and .0035lbs(25gr) with 88lbs of force.  Friction isn't considered at this point. 

Hope that helps
Bill

Standard deviation does not relate to psi.  Nor is it a product of extreme spread.  When we refer to SD we often are referring to a measure of variation in muzzle velocity compared to an average.  (I apologize if my definition is not altogether spot on.) The lower the SD here, the more likely each individual shot is likely to be very close to the average MV.  In other words, there is not a great deal of variation in MV, which is generally a good thing for accuracy. 

To illustrate, take a rifle that has an average MV of say, 800 fps.  It shoots a 20 shot string in which one shot was 790 fps and one was 810 fps and all the rest -- that is all the other 18 shots -- were one or two fps slower or faster than the average of 800 fps.  This rifle will have a very small SD even though it had a 20 fps extreme spread (ES).  Now take another rifle with an average MV of 800 fps and an ES of 20 fps, but in this case in a 20 shot string, half of them were around 790 fps while the others were all around 810 fps.  This rifle would still have an average MV of 800 fps and an ES of 20 fps.  Its SD, however, would be considerably more than the first rifle.

Now if the first rifle had shot only a 5 shot string, it would probably never experience its ES of 20 fps.  Only when it is shot a greater number of times -- its 20 shot string -- can the full ES of the rifle be seen.  The SD is very small but only a greater number of shots will reveal its true extreme spread.  It has the same ES as the second rifle but needs more shots to reveal it.

[PlanB]

Thanks for the replies y'all, I'll read um again in the morning, my brain punches the time clock about this time every day.

[PlanB]

Well my brain is working a little better this morning, y'all's explanations made sense to me except the 1st line that said psi and sd are not relative?