How much variance at chrono is normal

[jwhittin]

I say you should use your STD. What you do with it depends on your application.  For long range shooting you want to minimize it for better down range accuracy.  For competition, use it to ensure you meet PF.  
 

For a 127 gn bullet, min velocity is 985 for minor so add 2x32 for a min avg reload velocity of about 1050.  The data looks good.  You can adjust upward for accuracy and feel. 
 

Check your STD regularly and adjust as needed.  

Edited June 3, 2022 by jwhittin

[zzt]

The value of low SDs is that EVERY round will pass chrono.  My Open load is 169 PF.  The slowest load in typical 10 and 20 round chrono strings is 168 PF.  Once I had a round the chrono'd 167 PF.  Even if your SDs are double digit, the same applies.  Make sure every round in your 10+ chrono string makes PF.

 

I used to shoot 172 PF when I started Open, because I was told that was safe for the chrono station.  Every time I was chrono'd it was at 172 PF.  So that advice was malarkey. 

[Bakerjd]

On 6/3/2022 at 9:27 AM, zzt said:

The value of low SDs is that EVERY round will pass chrono.  My Open load is 169 PF.  The slowest load in typical 10 and 20 round chrono strings is 168 PF.  Once I had a round the chrono'd 167 PF.  Even if your SDs are double digit, the same applies.  Make sure every round in your 10+ chrono string makes PF.

 

I used to shoot 172 PF when I started Open, because I was told that was safe for the chrono station.  Every time I was chrono'd it was at 172 PF.  So that advice was malarkey. 

This is why I chrono 10 rounds then take the total of all 10 and divide it by 10 to get an average FPS. if it makes 5PF above what I need it's a good load. If it feels good I'm the gun it's the load I'm going to use. I went though this when switching to ramshot comp over the winter. Tried multiple powder charges amd found a few I felt like really testing. 2 ended up around 140PF and the third was 135PF. then tweeked the 135PF load some and played with recoil springs and ended up with my current load. Makes 131-132PF every time I test it or shoot a major. It's accurate and gun runs great. 

 

I also found that different powders drop drastically different in a progressive press using the dillon powder drop. For example titegroup I never could get to be as consistent as I wanted. Say I'm trying for a 3.5gn drop. I'd get anywhere from 3.0-3.7/8. Heard good things about RS Comp and tested it. Got it dialed in to +-.2gn. When I first started using it after initial setup I would check the last drop of every 100 rounds made for 1000 rounds. Every single one was 3.5gn. After 3300 rounds the very last round dropped at 3.5gn. I have since went down to 3.3gn and it's still the same excellent consistency. 

[GMM50]

When you get a good powder for your application, you'll see 10fps as the standard deviation,   we even see 1 or 2 for std.  Even with range brass.

Our goal is accuracy.  So 10 fps std can give 1 inch groups at 25 yards.

Edited June 6, 2022 by GMM50
typo

[davebenson]

On 5/30/2022 at 4:17 PM, fireman1776 said:

I finally picked up a chrono but was surprised to see how much variance I had in a ten round test. I’m wondering if my crimp needs adjusted. All rounds passed case gauge. 
 

127 conical brass monkey bullets over 3.6 TG. 1.150 oal. Shot out of a x5 legion. 

Mixed range brass can cause this.  As a test I took 25 mixed range brass and hand measured the powder and saw about what you have.  Add to that the press may not drop exactly to the tenth of a g of powder and there you have it. 

[jwhittin]

“The value of low SDs is that EVERY round will pass chrono.”

Completely wrong!  The SD measures the variation around (above and below) the mean.  
 

If you require every round to be at least the min vel to make PF, your loads will always be much hotter than statistically necessary.  Yes you meet PF, but with unnecessary recoil and higher cost.  There is nothing wrong with the approach, just pointing out the cost. 
 

There are many factors that affect SD and they don’t remain constant in your reloading process.  Using your measured SD as I described in the article, allows you to statistically determine a minimum Avg velocity needed to meet PF with a high confidence level.  Then you can adjust your load up for feel and accuracy.  
 

 

Edited June 24, 2022 by jwhittin

[Sarge]

I must be doing things right. At the Ohio match my three shots over chrono were 1508,1509,1512. 

[zzt]

On 6/24/2022 at 5:07 AM, jwhittin said:

“The value of low SDs is that EVERY round will pass chrono.”

Completely wrong!  The SD measures the variation around (above and below) the mean.  
 

If you require every round to be at least the min vel to make PF, your loads will always be much hotter than statistically necessary.  Yes you meet PF, but with unnecessary recoil and higher cost.  There is nothing wrong with the approach, just pointing out the cost. 

 

Rubbish.  My Open major load is 10.2 SWMP under a 115 HAP for 169 PF.  Min vel was 168 PF (1463).  Max vel was 170 PF (1484).  This load has MORE recoil and is LESS flat than the 10.6gr load I prefer.  I stay with 10.2 because the 10.6  load spills powder when the shell plate rotates.  10.8 is even softer and flatter, but more powder spills.