Seating depth and ES

[Tacticalrum]

I made some rounds .035 short to fit in the detachable magazine for my .338 Lapua. Shot them over the garmin and it decreased the ES to 5.0 and SD to 1.5. The original numbers were ES 14 and SD 6.8. Thoughts?

 

[spife7980]

Do it again

 

[ShtrRdy]

How many shots are you measuring to get ES and SD. As Spife suggested, try repeating the test.

 

[JB.IC]

ES too high

That's a good thought, particularly if he's seating the bullet deep enough where the bearing surface contacts that donut area. There can be enough variance in the bullet's specs where sometimes it touches and sometimes it doesn't when one is just close to it. Though in the OP, he did mention...

www.snipershide.com

I wanted to comment on the behavior of sample ES at small sample sizes. It suffers the same consequence as sample SD. As can be seen below, sample ES can be underestimated much like sample SD when the sample size is small. Depending on the population SD, convergence onto the population value can be as small as 10 observations or as high as 30 observations. Since you do not know the population standard deviation, you have no idea how underestimated the statistic might be. Worse, the plots below show “on-average” results. No one is testing their rifle 500k times just to estimate a statistic. The minimums and maximums in the table illustrate how disperse any one test could be. As one would expect, more observations shrink the range between the minimum and maximum.

TLDR:
ES at small sample sizes are unreliable.

View attachment 8480321View attachment 8480322

 

[Franko]

Sample size likely too small. Shoot 20 rounds and see how your ES and SD numbers hold up.

 

[RegionRat]

All of the above plus an additional background question. (well, really a few questions to stir your thoughts)

You mention the current loads have a jump of 0.035" .... does that mean you were at touch or jam before?
How would you describe the previous load's length in terms of the distance to the lands?

Did your "vertical" or group improve too, or just your velocity stats?

When running the comparison tests, were the previous rounds and the current rounds made of brass that was all prepped as one batch and did the latest tests include the old baseline during similar conditions?

Assuming temps on different outings are the same... How does your batch to batch velocity statistic normally run? Is it less than 1 SD on the average velocity, or does session to session vary by more than one SD on the average velocity?

These questions explore your level of experience with brass prep and neck friction. We will assume your headspace and primer seating depth and primer type were identical between the sessions, but if they were not than you still have work to do.