Decent SD but High ES

[Dave M.]

Hi
I have found a load that is giving me an SD of 8 and an ES 29 fps over 10 shots
It's grouping well yet I don't know why the ES is so disproportionate
Attached is a 9 shot group at 100m

You can expect the ES to be approx 4 times the SD for longer strings. A 29 ES and a 8 SD are perfectly aligned.

 

[Dave M.]

You cannot truly have a decent SD and high ES, small sample sizes yield variable answers. For a suitable sample size ES= 6 SD as reality across the long term which you will ultimately experience. Anyway it's the target that matters.

With the sample size we normally shoot, Litz states ES is generally equal to 4 SD (not 6). 95.44% of shots will lie within +/-2 SD (that means only 4.5 out of every 100 shots will be outside +/- 2 SD.

 

[CharlieNC]

With the sample size we normally shoot, Litz states ES is generally equal to 4 SD (not 6). 95.44% of shots will lie within +/-2 SD (that means only 4.5 out of every 100 shots will be outside +/- 2 SD.

True. 6SD is 99.7%. Pick the confidence level you like. The OP data does not align with either due to sample size and luck of the draw, my point being his inference was incorrect.

 

[Dave M.]

True. 6SD is 99.7%. Pick the confidence level you like. The OP data does not align with either due to sample size and luck of the draw, my point being his inference was incorrect.

Then we are saying the same thing. The OP’s SD is inline with the ES

 

[CharlieNC]

Then we are saying the same thing. The OP’s SD is inline with the ES

His title Decent SD but High ES is a wrong conclusion, which can easily lead to incorrect actions, which is the problem. There were numerous recommendations on things to try to correct a false-problem!!! Desiring a lower SD is one thing, no need to falsely imply when there is none.

 

[RegionRat]

If anything, we can say he has yet to see the whole story since 4XSD is 32 and 6xSD is 48, so by any habit he doesn't need to panic about the ES being 29 when his SD estimate is 8.

We all know the next time out his SD will swing higher or lower anyway.

 

[Dave M.]

His title Decent SD but High ES is a wrong conclusion, which can easily lead to incorrect actions, which is the problem. There were numerous recommendations on things to try to correct a false-problem!!! Desiring a lower SD is one thing, no need to falsely imply when there is none.

Precisely correct. Exactly what I was trying to say. A 29 ES with an 8 SD is perfectly aligned. There is nothing out of ordinary with this type of data and I’d be willing to bet that almost every 20 shot string he fired with this rifle and ammo combo with exhibit similar results.

 

[PracticalTactical]

Hi
I have found a load that is giving me an SD of 8 and an ES 29 fps over 10 shots
It's grouping well yet I don't know why the ES is so disproportionate
Attached is a 9 shot group at 100m

I have to ask the obvious what you are using to weigh powder and sort cases.

Also less obvious causes of velocity variation are chamber related... The length and diameter of your free bore will affect velocity spreads. The neck clearance also affects velocity spreads. If you think about this for a sec, that a bullet that starts into the rifling straight will always have the most consistent possible velocity via lowest possible force required to engrave the bullet.

Angular offset of the bullet increases the force required to engage the rifling. So when all your reloading is tight, look to the chamber.

 

[JFrank]

I have to ask the obvious what you are using to weigh powder and sort cases.

Also less obvious causes of velocity variation are chamber related... The length and diameter of your free bore will affect velocity spreads. The neck clearance also affects velocity spreads. If you think about this for a sec, that a bullet that starts into the rifling straight will always have the most consistent possible velocity via lowest possible force required to engrave the bullet.

Angular offset of the bullet increases the force required to engage the rifling. So when all your reloading is tight, look to the chamber.

The diameter of a freebore may vary slightly but the competition reamers we use now only have about .001 over bore so i just don’t see where the misalignment comes from.

 

[PracticalTactical]

The diameter of a freebore may vary slightly but the competition reamers we use now only have about .001 over bore so i just don’t see where the misalignment comes from.

Well, lets start with the idea that a reamer manufacturer always errors on oversize. Probably by more than you would like.

But the worst culprit is the engine lathe that cuts oversize. Since the body of the reamer is stiffer than the neck and throat, the deflection occurs in the neck and throat, which produces a trumpet mouthed hole.

People who are not involved in precision machining rarely grasp the significance of this point. And by precision machining, I'm not speaking of engine lathes that you can buy for $5000. I'm talking about upwards of $500,000.

I'm not suggesting that anyone "needs" a $500K lathe to chamber a barrel, but if they don't have one, they need to be more creative than the typical pipe fitter to produce a great result. This is why so few barrel fitters are of any note.

Few consumers have the tools or skills to evaluate their chambers, but suffer at the unknown of it and typically reduce the difference to handloading skills and components.

As for the optimistic 0.001 oversize... Take a bullet and measure the bearing surface with a micrometer, then open the mic by 0.001" and wiggle the bullet. Pay attention to how much angular movement there is... Then open another 0.0005 or 0.001" and see what you get. These differences will appear on your chronograph.

In this case we are discussing tolerances of someone who is somewhat competent and well outside of gunsmithing norms.

 

[JFrank]

Jim:

Well you said quite a bit but it doesn’t appear to be getting much traction, let’s take a look shall we? Now myself not being a machinist of superior intelligence I will refer to JGS shop tolerance of .0005 - that doesn’t seem excessive. Let’s say it real slow 5/10,000 .. nope still doesn’t sound excessive.

https://cdn.shopify.com/s/files/1/0062/6986/1957/files/Shop_Tolerance_JGS_Precision_Gun_Gunsmithing_Gunsmith_Tooling.pdf?16315775808108721187

Well, lets start with the idea that a reamer manufacturer always errors on oversize. Probably by more than you would like.

But the worst culprit is the engine lathe that cuts oversize. Since the body of the reamer is stiffer than the neck and throat, the deflection occurs in the neck and throat, which produces a trumpet mouthed hole.

spj
Next , we have reamer deflection creating trumpet chambers. that sounds pretty bad although you are the first one I have ever heard even whisper the T word so that just sends me to the curb with confusion, I may not recover until I ask JGS about their bending reamers.

People who are not involved in precision machining rarely grasp the significance of this point. And by precision machining, I'm not speaking of engine lathes that you can buy for $5000. I'm talking about upwards of $500,000.

I am certainly not involved in precision machining, how could I grasp such a concept without your superior intelligence. I don’t even know any gunsmith using a half million dollar lathe, do you ?

I'm not suggesting that anyone "needs" a $500K lathe to chamber a barrel, but if they don't have one, they need to be more creative than the typical pipe fitter to produce a great result. This is why so few barrel fitters are of any note.

What is a barrel fitter ?

Few consumers have the tools or skills to evaluate their chambers, but suffer at the unknown of it and typically reduce the difference to handloading skills and components.

Yes I’m lacking in skill so I let the spent brass and target evaluate the chamber. So far so good…

As for the optimistic 0.001 oversize... Take a bullet and measure the bearing surface with a micrometer, then open the mic by 0.001" and wiggle the bullet.

I had to think about this great parallel for a moment then realized that we don’t use micrometers to hold bullets, the bullets are held in a undersized neck by the length of the bearing surface so this wiggling bullet concept really isn’t valid imo.

Pay attention to how much angular movement there is...

Then open another 0.0005 or 0.001" and see what you get. These differences will appear on your chronograph.

I would love to see you prove that an extra .001 of neck chamber clearance can be identified on a chronograph. You would be the first to do so that I’m aware of.

In this case we are discussing tolerances of someone who is somewhat competent and well outside of gunsmithing norms.

You lost me on this one, are you saying your skill level is above the top gunsmiths or they are not competent and you are ?

 

[PracticalTactical]

SPJ, its obvious that your intent is confrontational in nature and not truly interested in answers other than to pick them apart further so I'll leave you to it.

I'm sitting within 60 feet of a firearms designer and 40 other CAD designers while looking out my office window to the shop with over $50M worth of CNC equipment as I type. I have overseen our research and development program for the past 13 years and started in aerospace manufacturing as a process engineer back in 1979. It's not like I'm unfamiliar with the subject.

Your last point is curious to me..."You lost me on this one, are you saying your skill level is above the top gunsmiths or they are not competent and you are ?"

In my original post, I was clearly referring to "gunsmithing norms"... but somehow you read "top gunsmiths"... I don't understand how you can make that leap.

I also thought I was clear in stating that a $500K lathe is not required to chamber a barrel accurately, but it does take someone with an advanced level of skill to produce consistently perfect chambers with an engine lathe. This requires an understanding of the weaknesses of such a system and the skills to overcome those weaknesses. It can be done, but that is the difference between a normal gunsmith and a top gunsmith. The devil is certainly in the details.

I can tell you that the few top gunsmiths that I have discussed chambering technique with walked away better for it and were quite pleased to say so.