Velocity change with powder position -- progress maybe

[earlmck]

Last time I posted on observations about the different velocities (or lack thereof) obtained with a "powder back" vs. powder forward position with some low-volume powder charges, I speculated that maybe the sectional density of the bullet was important. Seemed that all the loads we had found which appeared not to be position-sensitive had relatively high sectional-density bullets (a .250 SD 38/55; a .270 SD 30/30; and a .360 SD 43 Mauser), while my low sectional-density loads were showing considerable position sensitivity. So here's what I did: in the 454 Casull case I made up a batch of loads with my lowest SD .452 bullet, a 185 grain Lee using 8.5 grains of Green Dot powder. Then I made the same loading with my highest SD bullet, a 315 grain Lee. Here's what I found today:

Sectional Density = .129
185 grain, powder forward -- 1058 +/- 28 fps
185 grain, powder back -- 1249 +/- 19 fps

Sectional Density = .220
315 grain, powder forward -- 1005 +/- 6 fps
315 grain, powder back -- 1036 +/- 9 fps

Sooo -- huge position sensitivity with the very low SD 185 grain bullet; much much less position sensitivity with the kinda' mid-range SD 315 grain bullet.

I also made some loads with the 8.5 grains Green Dot and the 225 grain bullet I used in previous tests. But this time I stuffed in a wad of dacron to hold the powder back. 1220 +/- 22 fps -- not too good considering the previous "powder back" load had given +/- 14 fps. So my dacron seemed to increase my standard deviation of the load, and as you might guess accuracy was mediocre.

I think I'm learning something.

[jmiller]

This would have a lot more meaning statistically if you shot something like ten each and published the range and average of each load. It takes the "randomness" out of the equation. However, it's a very good subject to investigate. It's difficult to make a statement on a population of 1 data point. Doing more of each will shore up your findings.(Maybe you did do that and didn't share the findings?)

[M. M. Wright]

Looks like promising research to me Earl. I'm interested in your finding.

[earlmck]

This would have a lot more meaning statistically if you shot something like ten each and published the range and average of each load. It takes the "randomness" out of the equation. However, it's a very good subject to investigate. It's difficult to make a statement on a population of 1 data point. Doing more of each will shore up your findings.(Maybe you did do that and didn't share the findings?)

?? Sorry j... that was average of 5 shots and that little +/- figure? That is one Standard Deviation calculated in Excel spreadsheet.

[Ben_Rumson]

OK I'll stick my neck out... Heavy bullets seem to mitigate powder position sensitivity....

[earlmck]

OK I'll stick my neck out... Heavy bullets seem to mitigate powder position sensitivity....

Well, at least heavy-for-caliber. At this point in my little investigation that seems to be the most noticeable difference between those loads that are very position-sensitive and those that are not.

[JB]

I never did any in depth testing, but I always used pillow stuffing to hold the powder in place in many 454 loads. It did seem to help with consistency over the chrono.