Case Volume Variances? WOW.

[GT40MkI]

This may be old news/expected...I started to perform a ladder test with Outlaw1 last Thursday.

My OCD set in and I weighed Bullets, Cases and even the BR2 primers, and set 5 rounds in Winchester Brass and 5 rounds in FC 08 each at 42, 42.5, 43, 43.5, 44, 44.5, and 45 grains of Varget...

Results as follows (all 175 SMK, each group of 10 was weighed to be exact to .1 grain)
42
Fed- 2550
Win- 2512

42.5
Fed- 2552
Win- 2522

43
Fed- 2639
Win- 2565

43.5
Fed- 2665
Win- 2627

44
Fed- 2663
Win- 2641

44.5
Fed- 2739
Win- 2677

45
Fed- 2785
Win- 2720

 

[GT40MkI]

Re: Case Volume Variances? WOW.

Despite velocity, the obvious nodes were around 43-44 grains...

All rounds were loaded to 2.85 oal (just under ai mag length) and fired from a 22" barrel...

Should I repeat this test with 43-44gr in .2gr increments, or is the idea to have a suitable margin of error and use a quicker process than hand measuring and weighing every charge?

 

[cali_tz]

Re: Case Volume Variances? WOW.

Interesting. But can you post the ES for each?

 

[GT40MkI]

Re: Case Volume Variances? WOW.

Pardon my ignorance, but what is ES?

 

[EMorr]

Re: Case Volume Variances? WOW.

ES is the abbreviation for extreme spread, in most cases this used to describe the difference in between you fastest and slowest velocities of a certain load / string of shots. It can also be used to describe the difference in any unit of measure, i.e. powder weight, bullet weight, case weight, bullet length or almost anything that is measured.

 

[GT40MkI]

Re: Case Volume Variances? WOW.

Ah, thank you!...yeah a terminology thing, I'm used to referring to that as Standard Deviation or SD...

Without digging up the log book I believe it was less than +/-6 fps on all loads...

 

[memilanuk]

Re: Case Volume Variances? WOW.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: GT40MkI</div><div class="ubbcode-body">Ah, thank you!...yeah a terminology thing, I'm used to referring to that as Standard Deviation or SD...</div></div>

Not really. Standard deviation is a completely different calculation as opposed to extreme spread (more correctly termed ' range'.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Without digging up the log book I believe it was less than +/-6 fps on all loads... </div></div>

An SD of 6-ish is plausible. An ES of <12 for all the above loads in two different kinds of brass... is kinda questionable in my experience.

 

[cali_tz]

Re: Case Volume Variances? WOW.

ES is highest speed minus the lowest speed... just a measure of total variation in that string.

SD, at least as reported by my chron sw, is a SINGLE standard deviation assuming a normal or Gaussian distribution. For example my SD was 22fps... which means that 66% of my shots will be within 22fps as a group... 99% will be within 44fps, and 99.9% will be within 66fps of each other... assuming I recall my college statistics correctly. I don't know why the distribution would necessarily be Gaussian... something else to look into.

 

[memilanuk]

Re: Case Volume Variances? WOW.

cali,

Almost. ~68% of the shots will be within +/- 1 SD i.e. +22 or -22fps of your mean (average), ~95% will be within +/- 2 SD (+/- 44), and ~99.7% will be withing +/- 3 SD (+/- 66 fps). So with an SD of 22, you could (theoretically) expect the chrono to read anywhere within a 132fps spread (ES) from one string to the next. Not good

The distribution may not be strictly Gaussian... but its a pretty close fit for many naturally occurring phenomenon.