Finding accuracy Windows

[Bayourambler]

When working up a load for a rifle , I load 1 round at a time starting low and go outside fire it through the chrony, write velocity down, and examine case. Repeat.. As I increase powder charges I notice "flat spots" in velocity where a few heavier powder charges doesn't change velocity much, then all of a sudden at a certain grain, fps jump up. Is this same as a ladder test, are these "flat spots the same accuracy Windows as a ladder test on paper? I guess I need to find out on paper myself , but just wanted your take on this!

 

[R. Cobb]

As a rule in ladder tests, is to start low and work up the load. Never go below listed min. or over listed max. I weigh each powder charge when I'm ladder testing.
Myself, I make 5 to 10 rounds per charge weight. Then up the charge weight by .03 of a grain from the previous charge. Along with analyzing spent cartridges between each group test.
When I do ladder test, I shoot five groups of 5 shots each on paper. You want to see how they group. Don't adjust your scope. Then I'll go to my next load weight on a different target. Then compare the two, then onto my next load. You should see the groups get smaller then larger, by this you will see your best load.
With load testing you are concerned on groups, not hitting the bullseye. You can adjust your scope for that after you find the sweet spot and most of all keep good notes.

You will find while checking with a chronograph that your test loads at the same charge weight will vary somewhat shot to shot. Higher Velocity does not determine accuracy.
Hope this helps.

 

[243winxb]

increase powder charges I notice "flat spots" in velocity where a few heavier powder charges doesn't change velocity much, then all of a sudden at a certain grain, fps jump up.

I have read it may be a high pressure sign? But i would guess , there should be others signs also?

 

[Blue68f100]

Flats section if velocity can be caused by many things. (Powder charge, neck tension, bullet, powder, case volume different ...) You just can not tell by firing only 1 round. This is why it's best to shoot groups. Larger the groups the better the data will be. I use 3-5 shoot groups for quick screening only. Then I start loading larger groups around the area I want to look at.

 

[Bayourambler]

Sorry ,I guess I have worded it wrong. What I mean by "flat spots in velocity" is , for example, I shoot the following:
56 grains = 2600 fps
56.3 2615
56.6. 2625
56.9. 2635
57.1. 2705
I'm calling the 56.3 to 56.9 grain range "flat" because velocities do not hardly change.
then they will jump at a certain charge. Is this the same concept as a ladder test?

 

[pert near]

...Is this the same concept as a ladder test?

No. Chrono velocity windows & standard deviation are not necessarily good indicators of accuracy. The latter test supposedly finds a load(s) where the barrel vibration (harmonics) is at a consistent muzzle point (called a node) & therefore each bullets exits consistently. This is a good explanation here:
http://www.6mmbr.com/laddertest.html
You may also be interested in Dan Newberry's "Optimal Charge Weight" (OCW) techniques. His explanation is here:
http://optimalchargeweight.embarqspace.com
Developing that perfect load can be frustrating sometimes & fun other times. But it always ends up rewarding.
Have fun & keep up the experimentation!

 

[macgrumpy]

You are seeing the place where the case has expanded fully in to the chamber. As you increased the powder charge the pressure increased. At the lowest powder charge the case has not expanded completely in to the shape of the chamber (the shoulder angles or in to contact with the bolt face). When you reached the powder charge that pushed the case in to full contact with the chamber the case couldn't expand any more and the pressure started to rise more quickly. Mechanically and ballistically full expansion is desirable but the trick is getting the case to expand fully when you've reached the desired muzzle velocity, this will help to minimize the velocity extreme spread (ES) and standard deviation (SD). This isn't the only thing that dictates good ES and SD values but it helps, the correct powder burn rate will produce this velocity flattening in such a way that you will have a little room to produce the right velocity for you rifle.

 

[Laphroaig]

This is a good explanation here:
http://www.6mmbr.com/laddertest.html

This is the method that is typically referred to as a ladder test. The definition has been modified by many to indicate other things, which is OK as long as we define our methods.

I, for one, still do the one shot per charge wt. ladder test as a starting point. In theory, it is a method to point a shooter to potentially accurate loads with a minimal amount of shots fired by looking for "nodes", or "windows" where bullets group together even though the powder charge varies. Sometimes I get results that are in agreement with the theory, sometimes the results are inconclusive. So to answer the OP, the proof is how the shots group on paper. Barrel harmonics are the driving force behind an accurate load, not chronograph statistics.

I did check a recently chronographed ladder test I did for my 25-06 and 2 out of the three nodes I did further testing on did have velocities that seemed to level out in that region of charges. So maybe the two go hand in hand? I haven't been chronographing long enough to have a firm opinion.

 

[Bayourambler]

This makes sense to me. Well explained.

 

[spitballer]

As I increase powder charges I notice "flat spots" in velocity...

A chronograph is an excellent tool but it often generates more questions than answers IMO. The only thing I've determined with any certainty using a chronograph is that published load data for a rifle is apparently based on the first shot from a cold 24" barrel.

 

[hdwhit]

Bayourambler wrote:
I load 1 round at a time starting low and go outside fire it through the chrony, write velocity down, and examine case. Repeat..

That's essentially the same process I follow except that I load more than one round at a time.

Bullets can vary in weight and dimension and that can impact velocity. I once sat down with a box of 250 9mm bullets that were supposed to be .355 in diameter and ran each of them through a micrometer. I ended up with the diameter ranging from .3548 to .3562 with most clustered between .3554 and .3556. With a sample size of one round, there's no way to identify or correct for velocity variations attributable to variations in the bullet.

In general, I load ten rounds at each charge weight. Five will be fired through the chronograph with minimal regard to accuracy (except to ensure the chronograph doesn't die of "lead poisoning") and five will be fired with a focus on accuracy (still through the chronograph if it is practical).