Rate of fire and it’s effect on standard deviation

[ih784]

Good evening,

I shot two separate 5 shot groups. The first group was relatively fast at 100 yards. With an SD of 19.4/ES48. The second group was slower and shot at 425. This group had an SD of 10.7/ES of 33. I know barrel temperatures effect velocity. During load development or shooting in general, does rate of fire effect the SD of a “decent load?” Side note: my Chrony is older and doesn’t seem to be as consistent as it used to be.

 

[mikecr]

How warm was the barrel for first group, and how long between groups?

 

[ih784]

The first group had been cooling for about 10 minutes from a previous load. So, warm to the touch but just above body temperature. The second time I shot the identical load, the barrel had about a 20 minute cool down.

 

[jelenko]

I think there's not enough data to draw any conclusion. My understanding is the differences in SD/ES between the two 5 shot groups probably just falls within the normal distribution. I.e., you'd need to repeat the 'experiment' several times to get enough data to compare the two conditions. I don't know how many times - there are members here who do know.

 

[MikeMcCasland]

I think it can have an impact, but is probably powder dependent.

I routinely get fairly low SDs (around 5 FPS) across ~20-30 shot strings. That's running a single base extruded powder though.

IF you were to run something double based, and/or spherical, while also letting the rounds 'cook' in the chamber while waiting for your wind condition, I bet you'd see quite a bit of variation.

Lastly, SD on 5 shots is an indicator, but it doesn't really tell you much.

 

[ih784]

I understand and agree with what y’all are saying. It’s my understanding that “it won’t get any better” lol. The SD and ES will only go up. No real signs of pressure. I’m seated about .005 off the lands and starting to compress the load as is. Would seating the bullet deeper in the case improve my numbers? Obviously pressure will go up and I think I can afford to drop a couple tenths of powder (still be sub-moa) if needed. Everyone’s experience is greatly appreciated.

 

[3Sigma]

https://www.statisticshowto.com/probability-and-statistics/hypothesis-testing/f-test/

Run an F-test with alpha set to .05 or .01 You need the SD and group average to do so. It will tell you if it is statistically different to 95% (alpha = .05) or 99% (alpha = .01).

This is the best way I know with such a small group size. My guess is it will be null, meaning you cannot say if they are different or not.

My thought is time of day, cartridge temperature, chamber/barrel temperature, and measurement system (chronograph) are enough to make that much of an impact for such a small sample size.

Real question: What does the target say? Some of my best groups have been with SDs on the "bad" side.

 

[ih784]

I have three targets with 3-5 shot groups under an 1” across .4 grains. 27.8,28.0,28.2. All three are printing in the same spot (.5) above the POA. As the charge went up, the groups got tighter. The last two groups quarter and dime size. So, I’m thinking I could adjust the seating depth on my dime sized (28.2) load and tune the powder accordingly? I believe y’all when you say there’s not enough data. However, if I developed the data and it proved one way or the other... I’d have an idea of what I could do to help it along.

 

[mikecr]

From my experience 10min between action is the very worst timing. That is, barrels seem least stable there.
ANY other timing, faster or slower, will do better.
If you really want to see bad, shoot 1sht/10min. This is my cold bore development timing (because it's the worst).

 

[Laurie]

I've noticed several times in testing batches (with say rising charges) that the first shot on resuming after a break to let the barrel cool will be a velocity outlier giving that batch a larger ES.

I first became aware of this in testing different SR primers in 308 Palma brass where 17 rounds were loaded with each model of primer. A break was taken after 10 shots to let the barrel cool some. Shot 11 wasn't invariably the low MV shot of the overall string, but was in most cases. Always low and sometimes substantially down on the rest. Even where shot 11 wasn't low shot, it was always within the lowest few, never at or above the overall average.

It sometimes pays to record individual shot MVs instead of just letting the chronograph work out average / ES / SD and recording that alone. That way you can pick out any regular pattern like this one.

 

[Tempest]

A number of years ago, I thought of this very topic as well. Was proud of my ES/SD on my FTR loads for 5 shot groups during testing. One day I thought I would test it simulating an FTR match of around 25 rounds around 10 sec apart. Very different results.

Wind changes are still a larger problem for me.