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Extreme Spread and Standard Deviation


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What are good ES and SD? Is an SD of 30 good? Is that average? Is it lousy?

 

I have kept with 10 round strings simply because that is what my chrony records. I am using the Sierra 65gr SBT GameKing over 24.3gr of Ramshot TAC using CCI SRP and once fired XM193 brass - fired from a 16" AR. Loading on a Dillon 550 with Dillon dies and pm. The best I seem to be able to do is an SD in the high 20's and seem to average about 30. I am able to get mid to high teens with my handgun loads (both 9mm and 45 ACP using Berry's plated  bullets over BE-86 in mixed brass with CCI primers) all day long.

 

Would varying my powder charge help? Seating depth? Am I just as good as I can expect to get from the AR platform (my groups are good and have been able to get several 5 shot groups in less than 1.25" at 100 yards. My best group is just a hair under 1" at 100 yards.

 

Thanks.

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What are good ES and SD? Is an SD of 30 good? Is that average? Is it lousy?

 

I have kept with 10 round strings simply because that is what my chrony records. I am using the Sierra 65gr SBT GameKing over 24.3gr of Ramshot TAC using CCI SRP and once fired XM193 brass - fired from a 16" AR. Loading on a Dillon 550 with Dillon dies and pm. The best I seem to be able to do is an SD in the high 20's and seem to average about 30. I am able to get mid to high teens with my handgun loads (both 9mm and 45 ACP using Berry's plated  bullets over BE-86 in mixed brass with CCI primers) all day long.

 

Would varying my powder charge help? Seating depth? Am I just as good as I can expect to get from the AR platform (my groups are good and have been able to get several 5 shot groups in less than 1.25" at 100 yards. My best group is just a hair under 1" at 100 yards.

 

Thanks.

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There are many things you could do.... Are you married to that bullet/ weight? If not try a different round to see if your barrel prefers a heavier or lighter round.

 

Do some load development and try adjusting your powder charge by .1 or .2 increments. See what shrinks or increases your group.

 

There are way too a ton of things you can do. Search on the forum there is a ton of info on here.

 

Hopes this helps

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22 hours ago, CPD7119 said:

There are many things you could do.... Are you married to that bullet/ weight? If not try a different round to see if your barrel prefers a heavier or lighter round.

 

Do some load development and try adjusting your powder charge by .1 or .2 increments. See what shrinks or increases your group.

 

There are way too a ton of things you can do. Search on the forum there is a ton of info on here.

 

Hopes this helps

 

Thanks for the input, but I am happy with the accuracy I am getting. What I want to improve is the consistency of my muzzle velocity. If I cna cut by Es and SD in half, I will be happy.

 

With that being said, no I am not married to that bullet. I do want to stay in the mid 60's in terms of bullet weight and at 65gr. this one is right there. With an average group of around 1 1/4", it is giving me the accuracy I am looking for. What I am not getting is the consistency that I want. My assumption has always been that getting a good ES and SD are the results loading consistently (seating depth, charge weight, case length, etc...). However, I am planning on adjusting some of my recipe, like charge weight and seating depth, to see if that has any impact on consistency.

 

I am willing to change the bullet, if that will make a difference in the consistency of the muzzle velocity, but that would certainly not be my preferred approach. I will try trickling my powder charge first, although that is also not a preferred solution as I would like to continue to measure and drop my powder charge on-press.

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  • 3 weeks later...

What are your SD and ES numbers?   I really need to know both.  Imagine you have a 10 rd string; 9 rds are all 2800 fps exactly, and the 10th is 2900 fps: ES = 100, SD = about 30.    Provided the group was good enough, as defined as 10 rds, 3" @ 300 yds, this would indicate to me that there was an issue w. 1 round as opposed to all 10 and id examine the cause of that one round to give me another 100 fps.  (this is a simplified example)

 

In reality, you are more likely to get what are good SD numbers but still have high ES: I am more interested in the ES for a sample than I am of the SD.  If the ES is low, 20-30 FPS from 10 rds, the SD will almost always be low as well.  

 

so, if 5 shots in 1" at 100 is "good enough" in terms of accuracy I would first look at what primers you are using before I do anything else.  Try Rem 7 1/2's.  Next, are you trimming your brass?  You should as neck length variations will influence velocity and groups.  Try reloading brass you are SURE you have fired in that rifle if you want to really get serious as mixed brass, even FL sized, will very in shoulder length location and this will influence capacity, velocity and groups.  

 

Once you find the primer that the powder "likes best" you should see an instant reduction in velocity variations.

 

We could spend hours on this but this should get you started.

 

For NRA HP 1000 yds I want under 20 FPS ES.  For 3 gun .223, 30-40 is fine.

 

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