Seating Depth/Standoff Effects

[mehavey]

As discussed last weekend, I went back out to do a longer string of trials on seating depth effects on velocity.

I targeted 2,550fps w/ a 175SMK (match bullet/velocity using Quickload (adjusted for previous burn rate characteristics w/ this powder lot). I settled on the volume associated w/ 20 thousands off the lands and set Ba=0.410 to get an IMR4831 load of 53.8gr and (~)41,000psi.

With the exception of the first string, I ran ten sets of 3 rounds each at:
0.005" off lands -- (4 rounds including fouling shot)
0.010"
0.015"
0.020" (baseline)
0.030"
0.050"
0.070"
0.090"
0.110"
0.130" (and the limits of adjustment on my Redding Comp Die)

The physical difference between loaded cartridges was obvious:


The velocity effects (with the extended data) were also obvious, and fairly linear this time (even w/ a 3rd-order polynomial).



Accuracy was 1/2-3/4" at both 20 thousandths (somewhat expected), and 130 thousandths (unexpected).

Bottom Line: Ignoring engraving forces (yeah, I know) and assuming a well-behaved pressure curve associated w/ final velocity (yeah, I know again), the pressures due to increasing seating depth started at about ~40,500psi at 5 thousandths standoff, and increased to ~47,000psi out at 130 thousands standoff.

[FORKLIFT352]

nice work!
I'm loading 223 and been using the COL from the book.
But after shooting many rounds I was curious to get the
best length for my AR.
It seems that in this case better speeds are achieved
without sacrificing accuracy.

Sooooo how far should the bullet be from the lands?

[parttime]

Thank You Mehavey.

It is nice to see comparisons like this, it always gives me more food for thought since I am still new to this hobby.

I basically shoot sporting arms for both hunting and pleasure and not for precision out to 1000 yards. Your graph and work has reaffirmed my belief that I should not be so concerned about how close to the rifling my rounds are but more on ease of chambering and concentricity of the whole cartridge.

Your statement also shows that a rifle can have more than one "pet" velocity. Does finding that velocity depend more on seating depth or powder charge??

Your graph also showed that a seating depth of .050 had the narrowest ES, what was the precision of that load?

[FlyFish]

Very nice work, sir. Thanks for taking the time to put this together.

[wncchester]

"Accuracy was 1/2-3/4" at both 20 thousandths (somewhat expected), and 130 thousandths (unexpected)."

This is the kind of info we get from people who know what they are doing because they have checked it out themselves. Then we get 'experts' who proclaim best accuracy comes from bullet contact with the rifling and they "know that" because they read it somewhere from another dufus.

[serf 'rett]

As a newcomer to the reloading discipline, I’m wondering why the 0.050 wasn’t the most accurate with the closer velocity grouping; same weight bullet at same velocity, in theory, has best grouping. Wondering if the higher degree of separation is related to the specific rifle or some other factor (pressure curve difference, wind, shooter, etc)?

[Doodlebugger45]

Nice work indeed. I don't pay as much attention to seating depth as I do to finding the best powder charge. It's not real surprising to me that you found 2 sweet spots with the seating depth. It's fairly common to find a couple different sweet spots when I vary the charge weights, assuming I run the tests over a large enough window of powder weights. I guess now that I have found some pet loads for various bullet/powder weights for various rifles, the natural thing to do will be to tweak the seating depths.

[mehavey]

Your graph also showed that a seating depth of .050 had the narrowest ES, what was the precision of that load?

The cases were 4th-firing WW cases/neck sized w/ a Redding. All powder weights were "dead on" using the RCBS ChargeMaster/calibrated by checkweights (and weighed twice). Seating was done with the Redding Competition/Micrometer Die (no crimp), and standoff distances were established/measured with a Stoney-Point comparator to within 0.001.

On the other hand... I did not segregate cases by weight (probably offsetting all that precision above).

Given only three shots, the low ES on the 0.050" load was probably just the roll of the dice.

...wondering why the 0.050 wasn’t the most accurate with the closer velocity grouping

Grouping is dependent on barrel dynamics & resonances -- which change as pressures, velocities and initial bullet/throat/acceleration conditions change. (In other words, magic.) In my experience it's actually not often that smallest groups and smallest ES's (within reason) coincided.

[serf 'rett]

Thanks for the response. Trying to do my homework before I bail off into rifle reloading. The rifle stuff sure makes the pistol reloading look simple.
Thanks for posting your test.

[wncchester]

"As a newcomer to the reloading discipline,...Wondering if the higher degree of separation is related to the specific rifle or some other factor (pressure curve difference, wind, shooter, etc)?"

As a long time reloader myself, I used to wonder about that sort of stuff myself. But, since there are many opinons and none have any proof, I just accept that what is, is. And work with it that way.

It's true that three samples of anything is statistically irrelivant and five isn't much better. But thirty or so samples begins to give some valid info!

[Unclenick]

Mehavey,

It's been interesting to see how the decrease in expansion ratio as you seat the bullet out has corresponded to a velocity drop. Bigger than expected, and I sure wouldn't have counted on that, figuring pressure increase due to loss of gas bypass as the bullet ogive gets close to the lands would have neutralized it. But data speaks louder than theory. I'd expect it to depend on the powder's ignition characteristics, too. Something a little quicker, like Varget, might have behaved differently.

I'm not surprised that you found more than one seating depth sweet spot. These things are somewhat mysterious. They don't seem to care much what the powder charge is, perhaps because the bullet starts to move at about the same internal case pressure regardless of what the peak pressure is going to turn out to be. Berger has found their VLD's do beast pretty far back in some rifles, though they used to suggest people seat to touch the lands and develop loads that way, as that did best in some. The first post in this thread is a letter from them about this and showing some rifles did best with VLD bullets as far as 0.165" off the lands.

I'm used to finding one sweet spot with the bullet bearing surface about one caliber into the case mouth, but that would be around a COL of 3.270" in .30-06, and many throats are too short for that with this bullet. The 175 grain SMK was designed for 7.62 NATO, in which you can operate closer to the normal cartridge maximum COL than in the .30-06 case. Add 30-06 maximum cartridge length to that bullet's ogive length (0.710 in Litz's book) and you get 3.204". Below that, the bearing surface is full in the neck.

I stuck an angled Stoney Point (now Hornady LNL) gauge into a Garand that I'd recently chambered with a pull-through headspacing reamer, and got 3.238" COL with land contact. So seating to 3.218" COL would be 0.020" off the lands in that tight match chamber. I figure that's a worst case. The bullet I used measured 1.246" long, where Litz puts it at 1.240", which is within normal variance. I assume Litz measured more than just one, so that would be 3.212" COL with a bullet the length Litz lists.