Velocity Extreme Spread and Standard Deviation

[chkunz]

I now have a Magnetospeed chronograph and am pleased with how easy it is to use. So now I am collecting a lot more velocity data than I did with the optical type chronograph where I had to set up out in front of the firing line on the club range. Now that I have more data to look at, one thing that stands out is my pet load velocity extreme spread and standard deviation. My ES and SD are a lot larger than I expected and much larger than what the F Class folks report that they think are required for what they are shooting. I have an accurate rifle and shoot master scores on the NRA conventional (sling) prone 600 yard target. I do most of what would be considered good reloading practices like weighing powder charges but I do not anneal cases and measure bullet seating force, segregate cases by weight, etc.

If I knew what the most important factors were in improving ES and SD I could consider whether I was willing to go to the extra effort. What do you think the factors are for improving ES and SD in order of priority?

 

[dkhunt14]

I think the most important 3 things are Load, seating depth and bullet tension. At least I feel they are the most important. Matt

 

[savagedasher]

I now have a Magnetospeed chronograph and am pleased with how easy it is to use. So now I am collecting a lot more velocity data than I did with the optical type chronograph where I had to set up out in front of the firing line on the club range. Now that I have more data to look at, one thing that stands out is my pet load velocity extreme spread and standard deviation. My ES and SD are a lot larger than I expected and much larger than what the F Class folks report that they think are required for what they are shooting. I have an accurate rifle and shoot master scores on the NRA conventional (sling) prone 600 yard target. I do most of what would be considered good reloading practices like weighing powder charges but I do not anneal cases and measure bullet seating force, segregate cases by weight, etc.

If I knew what the most important factors were in improving ES and SD I could consider whether I was willing to go to the extra effort. What do you think the factors are for improving ES and SD in order of priority?

Case capacity is number 1 Then primer slection 2 Powder drop 3 Bullet slection 4 Bullet slection will make how you have to prep the cases .
But you will never get low SD and ES if the case capacity isn't close to each other. Larry

 

[bheadboy]

+1

when I weigh cases to =/_ .02gr , trim, check neck thickness and make uniform, weigh charges, set primer uniformly with Hart hand seater, seat bullets to same base to Ojive, size cases to .002 set back,

SD and ES are nominal on the preferred charge after workup.

that said, that load when used on PD, I do not go through all that, just the powder, trim sort cases, when I was competing , I did it all.



Bob

 

[chkunz]

I think the most important 3 things are Load, seating depth and bullet tension. At least I feel they are the most important. Matt

Thanks for the reply. By "load" I am not sure exactly what you mean, do I get the smallest ES and SD with the load that shoots the smallest group? If I am getting large ES and SD is that an indication that I do not have the most accurate load? Do I get smaller ES and SD with more or less bullet jump? Do I get smaller ES and SD when I increase bullet tension or when I decrease bullet tension? Again thanks for your response.

 

[chkunz]

Case capacity is number 1 Then primer slection 2 Powder drop 3 Bullet slection 4 Bullet slection will make how you have to prep the cases .
But you will never get low SD and ES if the case capacity isn't close to each other. Larry

Thanks for your response, I understand what you mean by case capacity but not sure I understand what you mean by powder drop unless you mean weigh each charge accurately which I am doing. Can you expand on the effect of bullet selection on ES and SD? Thanks again for your reply.

 

[BigBlueandGoldie]

What kind of SD and ES numbers are you seeing?

 

[savagedasher]

Thanks for your response, I understand what you mean by case capacity but not sure I understand what you mean by powder drop unless you mean weigh each charge accurately which I am doing. Can you expand on the effect of bullet selection on ES and SD? Thanks again for your reply.

Depending on the bullet some will shoot better jamming others shoot better jumped Jamming neck tennion is critical . Larry

 

[savagedasher]

What kind of SD and ES numbers are you seeing?

My competition case will have a ES of 5 Larry

 

[dkhunt14]

Thanks for the reply. By "load" I am not sure exactly what you mean, do I get the smallest ES and SD with the load that shoots the smallest group? If I am getting large ES and SD is that an indication that I do not have the most accurate load? Do I get smaller ES and SD with more or less bullet jump? Do I get smaller ES and SD when I increase bullet tension or when I decrease bullet tension? Again thanks for your response.

I have had loads with small ES that shot big and vertical at 1000 yards and loads that had bigger ES and they shot pretty good at 1000 yards. Bullet seating depth can go either way. Some guns and bullets might like touch and some may like it in the lands or jumping. Bullet tension also can change accuracy and ES. Same as load it could go either way. Bullet tension has to be consistent from case to case.

One thing that I have seen in most of my guns is they like the load near Max. Most of my guns also have liked it in the lands for seating. I shoot 1000 yard BR and that is where I do most of my testing. Matt

 

[chkunz]

What kind of SD and ES numbers are you seeing?

Good question. I am shooting an AR in 223 with a 26 inch barrel. My 75 Hornady BTHP pet load loaded for magazine length for 200/300 yard NMC stages shows ES 31 and SD of 9. My 600 yard pet load with 75 A MAX shows ES of 60 and SD 20. Both loads will shoot less than 0.3 inch mean radius for ten shot groups at 100 yards and shoot master class scores on the NRA conventional (sling) target at the full range. I have shot scores as high as 590-29X for a 3X600 match with the 75 A MAX load. So the ammo performance is a lot better than the ES and SD would have me believe. One could argue to not fix it if it is not broke but now that I have the data I would like to use the ability to measure ES and SD to improve my loads but I would like to put my efforts where they will do the most good. I am not willing to just do it all and hope it helps. I am not shooting F Class so I have an easier target to shoot.

 

[dmoran]

I have had loads with small ES that shot big and vertical at 1000 yards and loads that had bigger ES and they shot pretty good at 1000 yards. Bullet seating depth can go either way. Some guns and bullets might like touch and some may like it in the lands or jumping. Bullet tension also can change accuracy and ES. Same as load it could go either way. Bullet tension has to be consistent from case to case.

One thing that I have seen in most of my guns is they like the load near Max. Most of my guns also have liked it in the lands for seating. I shoot 1000 yard BR and that is where I do most of my testing. Matt

+++1..... well said..... I am in agreement on all points !.!.!
While I strive to get low ES when developing/tuning, lowest ES does not necessarily "rule the roost" on the target.
The vertical dispersion on the target, at the intended target distance, trumps all - IME
Donovan

 

[chkunz]

I have had loads with small ES that shot big and vertical at 1000 yards and loads that had bigger ES and they shot pretty good at 1000 yards. Bullet seating depth can go either way. Some guns and bullets might like touch and some may like it in the lands or jumping. Bullet tension also can change accuracy and ES. Same as load it could go either way. Bullet tension has to be consistent from case to case.

One thing that I have seen in most of my guns is they like the load near Max. Most of my guns also have liked it in the lands for seating. I shoot 1000 yard BR and that is where I do most of my testing. Matt

This is great information but it has me wondering as to how good our chronograph data really is. It is hard for me to have faith in an instrument that I can not calibrate. I did a lot of data analysis in another life and I knew a lot about the quality of the data that I was working with including the calibration techniques and results. Thanks again for your input.

 

[ShootDots]

I think the most important 3 things are Load, seating depth and bullet tension. At least I feel they are the most important. Matt

I am with Matt on this one, with 2 additions. I will give you MY experience, and it was with 2 / 7mm Shehanes and a "straight" .284.. Before I got towards the "upper-end", getting fairly close to max loads, my e.s.'s were over 20fps spread. Once I got near the "sweet-spot" of charge weight, my e.s.'s dropped significantly. ALSO, changing primers can make a fairly substantial difference. Having said that, the BIG ones are as Matt delineated, seating depth and bullet tension. It has been my experience that bullet tension has the greatest "pull" for setting up shot strings of widely varying e.s.'s..

 

[dmoran]

chkunz +++1.... indeed
Chronograph data can be subjected. Especially from small samples, like most all test/develope from (me included).
Donovan

 

[dkhunt14]

ShootDots included primers, which is also important. I don't know a lot about AR's but I know in a bolt action the firing pin spring can change the ES and groups. Matt

 

[DBailey]

chkunz +++1.... indeed
Chronograph data can be subjected. Especially from small samples, like most all test/develope from.
Donovan

+1.
I have trouble understanding how people believe that shooting one 5 shot group over a chronograph gives them their actual ES and SD numbers.

ShootDots included primers, which is also important. I don't know a lot about AR's but I know in a bolt action the firing pin spring can change the ES and groups. Matt

+1 again.
Consistent ignition of the primer is and always has been the number one factor in obtaining good groups and low ES numbers.

 

[msinc]

I believe you will be amazed if you anneal the cases...once upon a time {1990} me and a buddy decided we were going to get our rifles and ammo as accurate and consistent as we could. At the time, he was the one with the place to shoot and the chrono. We did pretty much everything possible to the rifles and ammo we loaded that we knew, read about and heard. The SD/ES was still no better than like 50. I had some cases that were pulling hard off the expander ball and tried annealing them. Next shooting session my numbers dropped to like 5. All were less than 10.
If all the necks don't have the same hardness/softness it doesn't matter where you set the dies, outside neck turn, seat the bullet, adjust so all have the same interference {case neck thickness}...the tension holding/releasing the bullet cannot be the same. The harder the necks are the more inconsistent they become tension wise.

 

[chkunz]

+1.
I have trouble understanding how people believe that shooting one 5 shot group over a chronograph gives them their actual ES and SD numbers.

I ran a 20 shot velocity string with the MS chronograph and looked at the ES and SD numbers using the first five shots vs. the first 10 shots and then all 20. there was a 2.5% difference in ES between the five and the ten shot strings and a 0.07% difference between the ten shot average and the twenty shot average. So for this one data point the ten shot string is ok.

I did a similar calculation with the SD for this same twenty shot string and got a 11% difference between five shots and ten shots a 0.6% difference between ten and twenty shots. Again ten shots is good.

I did this sort of calculation many years ago with group sizes and since then I always shoot ten shot groups for testing. Also, I also learned to use mean radius for evaluating groups rather than extreme spread.

 

[chkunz]

+1.
I have trouble understanding how people believe that shooting one 5 shot group over a chronograph gives them their actual ES and SD numbers.


+1 again.
Consistent ignition of the primer is and always has been the number one factor in obtaining good groups and low ES numbers.