What to tune first: seating depth or powder charge?

[Unclenick]

This is what Dr. Lloyd Brownell measured back in the 1960's. Note that the bullet is round nose, so the taper of the sides of the ogive is very gradual, so it has to seat a greater distance than a spitzer nose shape does for the same size gap between the freebore and the bullet to open up. Dr. Brownell believed that gap controls the amount of gas bypassing the bullet from the moment gas starts coming out between the bullet and the case neck and until the bullet jumps and jams into the lands, obturating (sealing off) the bore for the chamber.

Here's a smaller spitzer nose bullet, though. Note the mean pressure drops 20% for just being 0.030" off the lands:

But that all begs the question, how do you determine the seating depth at which the optimal powder charge will produce the best grouping. Item 3. in this old page on load development and ladder shooting describes how an old 8 mm Mauser with a shot out throat became the tightest grouping hunting rifle Somchem had ever tested.

This Tech Talk page by Berger appears to find the same thing happens when seating depth is correct.

Like Somchem and Berger, the usual procedure is to establish a charge, then look for best seating depth. Is there interaction? Yes. If you make a load that is jammed into the lands, you need about 10% less powder than it takes to reach that same pressure when the bullet is about 0.030" off the lands. However, the jammed load will not reach as high a velocity you reach when the load is adjusted to reach the same peak pressure with some jump. Why not? Peak pressure alone does not determine velocity. If two loads give you the same velocity with the same bullet, the average pressure in the barrel is the same. But with more powder you have more gas, so the muzzle pressure is higher. So, for the velocity to be the same with a heavier charge of the same powder, the peak pressure actually has to be lower. This means acceleration in the early part of the barrel is lower so the bullet is going slower just after the peak and makes it up later with the higher sustained pressure the bigger gas quantity achieves past the peak. But that means that even though the final muzzle velocity is the same, the barrel time for the heavier charge is, ironically, longer. So it can move you off a sweet spot.

If you were to load to keep the same barrel time, which is what you want to stay in the flat spot of an Audette ladder, then you will have to load for slightly higher velocities as you back off from the lands.

So, ideally, you would run a ladder at each seating depth, find the flat spot for vertical stringing, then shoot test groups with each flat spot load to see which one produced the overall smallest group. It's a bunch of shooting.

Here's an example of the interactions simulated in QuickLOAD by changing start pressure to approximate results in that 6 ppc plot.

H322 23.5 grains 2862 fps 1.087 ms barrel time, 100% (reference pressure of 42,756 psi), jammed into lands

H322 23.5 grains 2763 fps 1.234 ms barrel time, 78% reference pressure, 0.030" off lands

H322 24.4 grains 2862 fps 1.184 ms barrel time, 86% reference pressure, 0.030" off lands

H322 26.1 grains 3050 fps 1.087 ms barrel time, 105% reference pressure, 0.030" off lands​

Note how, at 0.030 off the lands, the powder charge needed to get the same barrel time back (last row) is both higher pressure and higher velocity than the jammed load was. The third row matches the velocity of the first row, showing the jammed bullet uses powder more efficiently.

 

[Yosemite Steve]

Unclenick, you always have great feedback! One thing that I missed in this data was a comparison of different powders. I installed Quickload and cannot get numbers that the program produces to match my results in the field. There is probably a noob factor but my results with Superformance and the 180gr SST gave me much higher velocities seated .010" off the lands than jammed at the same powder charge with milder pressure signs on the primers. The burn rate for certain powders will likely produce different results. My trials with Quickload were cross checking field data using IMR4064. The numbers were not even close.

Black Mamba I stand corrected to some extent. I know not yet to what that extent is but I stand there corrected.

I have been hoping for your input here.
Thank you!

 

[Yosemite Steve]

Unclenick, I would like to know what that graph looks like with a .010" jump.

 

[Unclenick]

How the graph would look with a 0.01" jump is a little tough to do exactly, but you will not be far off with a linear interpolation as the line is fairly steep in over that short distance. So if you figure 0.030" is 20% lower pressure, the linear interpolation says 6.67% lower than the jammed pressure, or 13.33% higher than the 0.030" pressure. While that won't be exact, the fact there are three or four thousandths in combined case length and bullet ogive tooling variations means it is probably as close as you could rely on.

Depends on the bullet and cartridge. The model isn't perfect. If you put 43.5 grains of 4064 into a .308 case with 57 grains water capacity with a 168 grain Sierra MatchKing seated to 2.800" COL, and fire it from the SAAMI standard 24" test barrel length, you will see it predict Federal's claimed velocity of 2650 fps within 2 ft/s. So the model can work well. However, what I do is remember that the author had to buy his powder same as you or I, which, in the case of the old IMR stick powders is only held within 5% of the nominal burn rate, and figure the other properties have tolerances too, and you realize it may not match Hodgdon's data. What I do, instead, is go back and forth between the burn rate and energy content until I get a match to Hodgdon's velocity and pressure for the maximum load, then look at the minimum and may readjust a little to get the best tracking. At that point, the predictions are coming pretty close. I've saved those powder changes and found they often don't work with a different chambering, so they clearly are not the only things I ought to be altering, but it does allow a tight fit to a narrow case.

 

[Yosemite Steve]

How then did I realize a 103 fps increase in velocity simply by reducing the COL by .050"? It was not a fluke because the deviation was small.

 

[Yosemite Steve]

Unclenick, that was a good read on the ladder testing by Randolph Constantine.

I think my next experiment will be to shoot a known happy load at different seating depths from a .030" jam down to a .150" jump to see how it effects velocity and to find the best group. I am very curious to see what happens. I think I will do this test with any other powder I try as well to see if it has an effect on seating depth as well as velocity nodes. So far I have sweet spots at same velocities with different powders. I must eliminate variables to be conclusive.