Difficult Tune with 6BR

[Patruck825]

Morning Gents,

I'm new to 6br, but not to reloading or shooting in general. All of my previous barrels in other calibers have had very pronounced flat spots in velocity that indicated my nodes.

However with this 6br barrel, it seems nearly every load shoots well, but there are no flat spots in velocity to indicate a node. Here is a ladder test i shot the other day.

26" Hawk Hill - 105 hybrid

H4895:

28.8 - 2810, 2817, 2814 SD:3.6
29.0 - 2831, 2826, 2821 SD: 3.1
29.2 - 2847, 2847, 2841 SD 3.4
29.4 - 2856, 2858, 2851 SD:3.6
29.6 - 2881, 2861, 2867 SD: 10.2
29.8 - 2882, 2878, 2895 SD: 8.8
30.0 - 2897, 2907 SD:7.0

I picked 29.3, loaded up 10 shots and had an SD of 3.8, it shoots well enough to keep it there. But i am just curious if anyone else has experienced this lack of flat spots with the "ultra easy to tune 6BR"

 

[Toby Bradshaw]

Tune is determined by group size and shape on the target, not by chronograph readings. How does your 6BR shoot?

 

[Patruck825]

Tune is determined by group size and shape on the target, not by chronograph readings. How does your 6BR shoot?

There are a couple different schools of thought on this. One is the way you mention, the other aims exclusively for low SD and ES spreads and that usually translates to the best groups. It’s worked very well for me in the past.

It shoots well, under a half inch at 100 yards, unfortunately I don’t have access too longer ranges reliably enough to test on. I have matches to prepare for and can’t wait a month or two for an invite out to extended ranges.

 

[Toby Bradshaw]

There are a couple different schools of thought on this. One is the way you mention, the other aims exclusively for low SD and ES spreads and that usually translates to the best groups. It’s worked very well for me in the past.

I don't own a chronograph, but I do know how to tune a BR rifle.

I don't know a single winning BR shooter who tunes with a chronograph without looking at group size/shape. Maybe I need to get out more.

 

[Texas10]

Toby,
I am mostly a novice at this sport, but I thought I knew some of the basics. However now I read your target notes and am thoroughly confused.
Please school me on the "touch" dimension you write on your target. 1.754 is your touch, and you use 1.737 as jam but to me that would be a .017 jump.
Or am I not interpreting your notes correctly?

 

[Toby Bradshaw]

Toby,
I am mostly a novice at this sport, but I thought I knew some of the basics. However now I read your target notes and am thoroughly confused.
Please school me on the "touch" dimension you write on your target. 1.754 is your touch, and you use 1.737 as jam but to me that would be a .017 jump.
Or am I not interpreting your notes correctly?

The seating depth number is the bullet seater stem length on the Wilson die, so a longer measurement moves the bullet farther from the lands.

 

[Patruck825]

Well I got a chance to ladder test at 550yards. The load I picked using my chronograph technique ended up being the same one that stood out in the ladder teat.

Here’s a pic, one cold clean bore shot and then 3 to follow up. Orange dot is a 2” sticker.

 

[AckleymanII]

Patruck, that looks typical for a barrel that is clean, with some kind of residual solvents left in the barrel. Try pushing two patches saturated with lighter fluid through the barrel when you are through cleaning.

 

[Patruck825]

Patruck, that looks typical for a barrel that is clean, with some kind of residual solvents left in the barrel. Try pushing two patches saturated with lighter fluid through the barrel when you are through cleaning.

oh yes, I’m aware. That result was expected just explaining the flyer.
I do always patch the bore dry after cleaning. The first shot on a clean bore always chrono a a good 60fps slower.

 

[AckleymanII]

Maybe fouling characteristics of the powder you are using.

 

[Patruck825]

Maybe fouling characteristics of the powder you are using.

I think that a clean bore is much like running a car without oil, there is going to be a lot more friction, hence the slower speed and POI change.

 

[Gabe22BR]

Explain the horizontal please..

 

[Toby Bradshaw]

Well I got a chance to ladder test at 550yards. The load I picked using my chronograph technique ended up being the same one that stood out in the ladder teat.

Below are your chronograph data. Where is the "flat spot"? And since a ladder test gave you a result that produces good groups, in what way is your 6BR hard to tune?

 

[Patruck825]

Below are your chronograph data. Where is the "flat spot"? And since a ladder test gave you a result that produces good groups, in what way is your 6BR hard to tune?

View attachment 1056822

There is no flat spot, there is however a charge range with Lower SD's than the rest, i loaded in the middle of that charge range.
When i say difficult, i only mean there is usually a very clear flat spot in velocities, which were not present with this barrel.

The load i chose and the one on the target, is 29.3.

 

[Patruck825]

Explain the horizontal please..

I dont really think it needs explaining as its well within what i consider acceptable, but here are some factors to consider.

This is not a benchrest rifle, and the shots were taken prone off a bipod with a rear bag, not a sled.
It was 90 degrees and mirage was very present.
Wind.

 

[Gabe22BR]

Mmm, maybe I missed it?? Clean bore?
I get these after 30rds down the bore..

 

[Toby Bradshaw]

There is no flat spot, there is however a charge range with Lower SD's than the rest

You might wish to familiarize yourself with the confidence interval around estimates of standard deviation with tiny sample sizes. Here's an online calculator that will show that all of your estimates fall within the same 90% confidence interval: https://www.graphpad.com/quickcalcs/CIsd1/?Format=SD

Edit: here are the standard deviation data (blue) and 90% confidence interval bounds (red) shown graphically. All of the standard deviation 90% confidence intervals overlap across charge weight; i.e., none of the standard deviation estimates are statistically distinguishable from one another. Of course the reason for this is that standard deviation cannot be computed with confidence using such tiny sample sizes (N=3 in this case).

What you have actually found is that the 6BR is ridiculously EASY to tune -- hence its popularity. All that your chronograph data show are that muzzle velocity increases linearly with powder charge, and that there is no significant variation in SD across charge weights.

How about posting a photo of your ladder test?

 

[Patruck825]

You might wish to familiarize yourself with the confidence interval around estimates of standard deviation with tiny sample sizes. Here's an online calculator that will show that all of your estimates fall within the same 90% confidence interval: https://www.graphpad.com/quickcalcs/CIsd1/?Format=SD

That is something i am not familiar with, but interests me. How can one accurately predict the tendency of a load with a good SD to stay consistent like that? i take it 90% is good.

I am not sure how to use the tool, it says to enter the SD's and N, what is N?

 

[Patruck825]

Mmm, maybe I missed it?? Clean bore?
I get these after 30rds down the bore..

sorry, misread your post, edited the original to respond.

 

[JFrank]

Below are your chronograph data. Where is the "flat spot"? And since a ladder test gave you a result that produces good groups, in what way is your 6BR hard to tune?

View attachment 1056822

I was just going to say that I'd be loading that for repeatability