Question for OCW fans

[Bill Daniel]

Working up loads for 6.5 Creedmoor. Load A 1.103” Load B 0.574” Load C 0.581”
Load D 0.581” Load E 0.648” Load F 0.923”
One hundred yards.
Is the node between A and B or is it around C?
Thanks
All the best,
Bill

[Rimfire5]

First let me say that your groups from B through D are very good but, from my perspective, are just about the same.
You didn't say what the charge differences were between each of the loads (was it 0.1 grains, 0.2 grains or more).
I would assume that the seating depths or trim lengths were the same for each of the loads.
You also didn't say if you were shooting from a fixture or from rests.

Some manufactures who use fixtures to create test targets (e.g. Les Baer with two different rifles in .223 and .308) often shoot two 5-round groups from a fixture and they aren't as similar with the same ammo from the same fixture as the variations you show when varying charges from Load B through Load D.

That said, I would explore further from half way between load A and B to half way between load D and E (if your powder charges were more than 0.1 grain apart).

A difference of 0.007 in group size is essentially the same group size between Load B and Load D, especially for a small sample (less than 5 groups) (Even if you are using group averages, that small a difference could be the result of one good or one bad group used to calculate the average).
If you shot 4 or 5 groups of each load with a standard deviation of 0.05 or less, you can begin make a conclusion.

If you shot one group with each load, I hope you realize that you are assuming that 'shooter induced variations' are not impacting your results. I don't trust my own results with single groups because of my 'shooter induced variations'.
With one group in each load, even with no 'shooter induced variations', I would say you have an "indication" but don't have the basis for a firm conclusion.

The good thing you have going for you is that you seem to be able to shoot consistent group sizes across a pretty wide range of loads. That also says something about the accuracy of your rifle in that range of charges.

Have fun exploring your rifle and focusing in on what powder, charge, bullet weight, bullet type, and seating depth provides the best results.

[Bill Daniel]

Thanks Rimfire5:
The loads vary by 0.4 grains. They start 2 grains under published max(average of three sources). The case prep is the same. These are single three shot groups. The goal is to narrow the node search. Those loads, assuming there are more than one, will be tested at five rounds and the best with a ten round test.
The rifle is benched and on sand bags.
Thanks again for your advice.
All the best,
Bill

[sparkyv]

Looks to me to be between C and D. That's where I'd focus my 300yd 5x confirmation rounds. Best I can say without images, as the OCW involves more than group size.

[Bart B.]

People who have shot hundreds of 5-shot groups with the same load in a given firearm see a 3X to 4X spread in group extreme spread sizes. 3-shot groups can have a bigger spread. First one fired can be anywhere in the size range.

Sometimes caused by variables in the ammo and/or firearm. Other times by variables in themselves. Both can combine, too.

25-shot groups are required when at least a 90% statistically confident level is needed for the shooting system accuracy. System = ammo + rifle + human.

[Unclenick]

Bill Daniels,

Since you didn't provide any other details (charge and bullet weight, case and primer, and more importantly, the number of shots per group), I am assuming, from your cartridge, that the 0.4-grain steps are about 1% of charge. That's should be OK, but I would also try 0.3-grain steps to confirm what you found. I would also add an earler and a later step so you can confirm the pattern. You also didn't say whether your numbers are group sizes or group center locations, but I assume the former from the context. The OCW system is based on center location rather than group size because of the statistical possibility of a random outlier getting into an otherwise good group.

Based on those numbers being group sizes, I would agree that C is the midpoint, as B, C, and D are essentially the same. If A and E have two shots closer together than BC, and D are wide, but one outlier, I would consider all those groups may be the same and more shooting is needed to evaluate them based on group size. If you have access to a chronograph, see if there is a velocity flat spot (velocity not changing with the charge increments) associated with B, C, and D. That would be a confirmation you are in one or another type of node (velocity or barrel distortion timing).

[Nathan]

If that is me loading, I’m going to center around C and cut my charge increment in 1/2. You will probably be shooting B again this way which will clarify if B was a fluke or just a bit better....the exciting thing is you seem to have quite a range of good, unless you are down to 0.1gr increments already.

[Bill Daniel]

Thanks fellows.
Unclenick the bullet is a 142gr SMK, the powder H4350, primer Fed GM210M, brass Seller&Bellot once fired in my rifle.
Rounds 1,2&3 37.4gr, 38.1gr, 38.9gr.
Next 3 rounds (A) 39.6gr
Next 3 rounds (B) 40gr
Next 3 rounds (C) 40.4gr
Next 3 rounds (D) 40.8gr
Next 3 rounds (E) 41.2 gr
Last 3 rounds (F) 41.7gr
I am weak on geometry and groups A and B had two of the three shots touching but eyeballing the center of the group to the center of the bullseye the measurements are:
A 1.28”
B 1.43”
C 1.30”
D 0.73”
E 1.59”
F 0.78”
If it matters B had a group size of 0.574” and C&D 0.581” the others are in the original post.
I have not chronographed the loads yet until I found the most accurate.
Again many thanks for your advice!
All the best,
Bill Daniel

[Nathan]

Why are your charge increments not equal? That kind of jacks with the OCW results....

Did you shots round robin?

[Bill Daniel]

Hey Nathan, that last charge should be 41.6 grains so each increase is 0.4 grains other than the first three sighters.
All the best,
Bill

[Nathan]

Oh, now I see that better....sorry.

Are you shooting the groups round robin? This is really a curiosity thing.

[Bill Daniel]

Nathan, the groups were shot round robin. One shot from each load at its target till all three rounds of each load fired.
Thanks,
All the best,
Bill

[Nathan]

Thanks for the Q&A.

I say, center around C and cut the charge increment to 0.2gr. So,

40.0, 40.2, 40.4, 40.6, 40.8 This should show a pretty good optimal charge.

Have you varied seating depth yet? Where are you with that?

Also, in this accuracy range, everything matters. How consistent is your CBTD? Have you removed your cocking cam from your bolt and confirmed case fit to rifle?

[Bill Daniel]

Nathan, thanks for your interest and advice. I have not tinkered with seating depth yet as I have wasted many bullets chasing rabbits with three round data.
At present the bullets are seated at magazine max which is about 0.02” off the lands on my rifle.
This rifle started as a project to assemble a sub MOA AR 10 type semi- auto and has become a hobby. I have used quality parts and have tried the accurizing tips I have found and by trial and error am seeing improvement. A major issue is the violence a cartridge is subject to in the cycling of the action. The force of the return spring acts like an an inertial bullet puller to move the bullet (up to 0.02” with some bullets). At present by not turning the necks on these Seller and Belloit cases and using SMK 142 bullets with a prepped case neck 0.005 below finished cartridge I have decreased that jump significantly. Now I am seeing if this has helped in accomplishing the goal, sub MOA.
What is CBTD?
Thanks,
Bill

[Bart B.]

Statistically, all 6 groups are equal in my opinion.

I've shot dozens of 3 shot groups in a day and a 5X spread in group size is normal for a given load.

[Unclenick]

A & B, with two shots touching, should be tried with the wide shot omitted to see where the centers fall. If they are still about in the same place as they are now, then B may be a node center by OCW criteria rather than C.

Given what you are spending on bullets, you might want to drop $12 on a copy of the OnTarget precision calculator software. You just take phone photos of each target and transfer them to your computer and use the software to open the images and to locate the point of aim as well as each bullet hole on the target. It then figures out the group center for you and where it is located relative to the point of aim. Another version of the software, TDS, adds the ability to print targets with bar codes it can read from a scanner and then automatically do the placement and calculating as well as allowing you to combine targets into larger groups. However, it costs more.

[Nathan]

What is CBTD?

Case Base T shoulder Datum circle....problem is gas guns don’t like it when you get them tight. On gas guns, I just run fired -0.003” since they have a spring ejector pushing the round forward.

[Bill Daniel]

Nathan, I set the shoulder back 0.002”.
All the best,
Bill

[Bill Daniel]

Unclenick, the On Target app. looks great thanks.
Bill

[Bart B.]

Originally Posted by Nathan

Case Base T shoulder Datum circle....problem is gas guns don’t like it when you get them tight. On gas guns, I just run fired -0.003” since they have a spring ejector pushing the round forward.

In all rifles, the firing pin pushes the cartridge forward to the chamber limits before the primer fires. That let's the primer cup be dented at least 15 to 20 thousandths to compress its pellet so it detonates.

Consider what happens when a 2+ ounce firing pin pushed near 20 fps by a 25 to 30 pound spring force strikes a 1 ounce cartridge.

Rimless bottleneck cartridge case shoulders are often set back a few thousandths or more as they slam into the chamber shoulder. Then the round fires.