seating depth increments

[Shootin4fun]

In almost everyone of the 20 plus rifles I have done load development for the preferred seating depth changes with the charge weight. I do not believe it matters if you find the OCW or Optimum seating depth first because when you figure out the first one and then the second one you really need to tweak the first again to see if you can make it better. When you are talking about some the accuracy we are chasing, small changes in pressure can make a decent difference on accuracy. The ladder test and OCW, if done correctly, are trying to find a wide node of acceptable accuracy.

Yes, I believe this to be true in the sense that there is an optimum depth for different charge levels. As such, there is always some tail chasing involved unless I just get lucky and start with 2 variables that work together, give good accuracy, and don't show pressure signs. So there is significant trial and error involved.

Mike proposes that seating depth has a much greater affect on grouping than charge weight steps. Others claim the opposite. Interesting, in the Berger article they say "This sweet spot is a band .030 to .040 wide and is located anywhere between jamming the bullets into the lands and .150 jump off the lands." .030 - .040 wide!

On a related note, sample size- for me anyway, working with sporter barrels, group size may vary due to shooting technique not being perfect all the time. No way could I do a string of 1 or 2 shots at each charge level as some suggest in order to find a vertical plateau. I shoot minimum 3, max 5 at each charge level. Unless it's cold and cloudy out, I have now taken to using ice bags in order to compress the time for barrel cooling or else I just can't get enough testing done in one day.

 

[Shootin4fun]

Your case does not hold the pressure the chamber does. Because you are giving your bullet more and more of a running start, your pressure will decrease when seating deeper, at least within the range of normal seating depths (.00" off to approx. .250" off)

To your original question, I prefer increments of .015" and I am looking for low Extreme Spread and Standard Deviation in velocities as much if not more than grouping

Thanks for posting this chart, Woods! For a long time I have been operating on the thought that deeper seating = greater pressure. This pretty much dispels that. I envy your single digit SDs. I have only have a couple of load sthat holds that. And I wonder, what makes the 1 in 4 that isn't in the same hole or cloverleaf as the others go where it does...is it me or the load? Is it that I haven't nulled out the barrel harmonics completely? With a ES of 8 and SD of 4, one would think they should all be in the same place, not make it a 1 MOA group unless it's or both of those things if there was no wind. Drives me nuts!

 

[Mikecr]

My full seating gets me to a ~15thou rough CBTO.
In each case, final tweaking of seating shapes grouping. This would be within a +/- 5thou window(or less).

A long time ago I figured out that seating is the single biggest adjustment to great results.
I have yet to figure out why.
But this is what I've observed, and each has been observed by others here & there:
-If you were to do full seating testing at a mid charge, picking your rough best, and then load develop with that, repeating full seating testing would get you right back to the same CBTO.
-If you did the same again, but with different powder, faster or slower, you would still end up at the same rough CBTO.
-If you did the same again, but for a different node, lower or higher, you would still end up at the same rough CBTO.
-If you did it again, but 1500 rounds later on the barrel, you would still end up at the same rough CBTO.
With all this, seating seems more of a prerequisite than a broad 'tuning aid'.
For PB BR shooters with razor edge tunes, fine tweaking of seating provides for renewed group shaping, just like I see. This seems working around a detrimental trend in tune rather than actual tuning.
Or is it?

Passing more tests then I've seen otherwise is the notion that seating most affects pressure peak TIMING. That is, timing of the ping which travels back & forth the length of the barrel (OBT folks understand). This makes sense as there are multiple seating nodes, regardless of MV.

A powder tune is built of all things affecting burn rates, which vary as the bullet travels. This eventually gets you to where a gun/barrel is pointed on bullet release, and how consistent this ends up, but it's an averaging sum of many things. It is finely adjusted, right to the kernel of powder if desired(applied to the entire chamber/bore length area), and so this adjustment manifests as FINE.
If powder is not enough result-wise to overcome bad pressure peak timing, then seating raises it's head as prerequisite to best results. This passes tests.

I refer to seating as the COARSE adjustment, and all my life I've calibrated equipment, coarse, then fine. Meet that seating prereq, then very precisely dial powder into the best with it.

 

[Shootin4fun]

Thanks for your insights Mike, they're helpful.

If CBTO means cartridge base to ogive, then that is what I inherently do as well. Once I find an optimum CBTO for a given bullet, I use that as a constant when I change powders, primers, or headstamp. I gotta hold SOMETHING constant and this seems to work.

I recently did an experiment wherein I changed a working CBTO, that being Barnes recommended OAL that is .120 off of lands, to seating .050 off the lands in a particular rifle as is the general rule with Barnes and happens coincide with their recommended OAL in another rifle, and my groups totally fell apart. So I use their recommended length in both rifles, but one happens to be .050 off, and one .120 off the lands. Interesting. In addition, I have found that Barnes usually produce the best groups when being pushed to near max or max charges. Sometimes they change their published max though, as has been the case with the 7mmRM, and I followed their advice which led to longer brass life and greater stability but did require more tinkering with charges. They went way down, then back up some over a 3 year period.

 

[woods]

So there are still 2 adjustments to be made, velocity and seating depth. What I mean is that if you do a seating depth test first at a medium/safe powder charge first, you still will need to do a powder charge test.

For example, I recently rebarreled a 22-150 with 1 in 8 twist and want to shoot 75 gr SSII's (hunting here not benchrest). I want to use RL17 since I have lots and it has performed admirably for me in many rifles. The 75 gr bullet should shoot close to 3200 fps and that is fine with me but what powder charge will give me that? Loads like this are not in the manuals and manual data is often very different from what you get in the field.

1. So I can guess at what would be a medium load; 35 gr, 36 gr and load 4 at each seating depth; .030", .045", .060", .075", .090" off for a total of 20 shots. Say .060" turns out best and now I need to find velocity (don't want to shoot a 22-250 at 223 speeds) and the 35 gr only produced 3000 fps.

So I can now do a powder charge test of 4 at each level; 35.3, 35.6, 35.9, 36.2, 36.5, 36.8, 37.1 (I believe that you should approach max gradually). That is 7 loads at 4 each (3 has never been a good indicator for me) for an additional 28 shots in order to reach 3200 fps.

2. My method and actually what I did today was to load 1 load at each powder level

34 - 2886 fps
34.3 - 2886 fps
34.6 - 2904 fps
34.9 - 2962 fps
35.2 - 2996 fps
35.5 - 2996 fps
35.8 - 3051 fps
36.1 - 3079 fps
36.4 - 3125 fps
36.7 - 3182 fps
37.0 - 3185 fps

looking for pressure signs

Now I know I am safe and at the velocity I want with 37 gr and can do a complete seating depth test at that powder charge level.

How would doing a seating depth test first accomplish anything different except take more shots?

 

[Buckskinner]

Well if your just looking for a certain bullet velocity, then yeah, what's the difference... Most are looking for accuracy over velocity, so the coarse then fine makes sense and also stings as I've been doing it opposite for quite a while. And that is seeming to explain a lot of my past experiences...

But why shoot that 11 shot string from 34gr to 37gr if just looking for a near 3200ft/sec velocity. You could have done that in 4 or less without a problem.

 

[woods]

Because I had no data and started low so I would not get in trouble. I suppose it was misleading to say that in the example of doing a seating depth first I would pick 35 or 36 gr when if I were being super careful I would have chosen the 34. So then I would have needed the 11 increments

 

[Mikecr]

So there are still 2 adjustments to be made, velocity and seating depth. What I mean is that if you do a seating depth test first at a medium/safe powder charge first, you still will need to do a powder charge test.

Yes, that's right.
I'm suggesting best seating depth is independent of either velocity, or best powder load.
As far as concerns about barrel life & seating testing; I do the testing during brass fire forming, with both prerequisite to credible load development.

 

[FURMAN]

Yes, that's right.
I'm suggesting best seating depth is independent of either velocity, or best powder load.
As far as concerns about barrel life & seating testing; I do the testing during brass fire forming, with both prerequisite to credible load development.

Mike does bring up a good point, I sadly had not considered. Loads will respond differently to virgin vs fired brass so it may be more beneficial to go ahead and get seating depth out of way.

 

[woods]

So

If the seating depth preference is a constant, does that mean that one can choose any powder charge and have the same accuracy as using that seating depth at any other powder charge? Is there a charge/velocity that will also yield better results than any other charge/velocity?

Given that velocity varies with seating depth, i.e. decreasing with deeper seating, can one find better accuracy/ES/SD by slight decreases or increases in powder charge with seating depth being constant?

What would your parameters/methods be for choosing a powder charge after you find the seating depth indicated?

 

[Shootin4fun]

Yes, that's right.
I'm suggesting best seating depth is independent of either velocity, or best powder load.
As far as concerns about barrel life & seating testing; I do the testing during brass fire forming, with both prerequisite to credible load development.

Yes, good idea if you buy raw bass. I do that more now but when I first buy a rifle, I buy a hundred rounds or so of live ammo to break it in and just reload that, so it's already fireformed and ready to reload. It also guarantees that primer pockets are deep enough; I've nightmarish experiences with Winchester unprimed brass (243) where the pockets had to be machined to the proper depth; what a pain.

 

[Mikecr]

If the seating depth preference is a constant, does that mean that one can choose any powder charge and have the same accuracy as using that seating depth at any other powder charge?

In your mind, separate seating from powder, and understand that they both, independently, affect overall results.
Two paths:
1. Pull an untested seating number out of your butt, powder develop with it, then go back to fiddling with it. But full seating testing will not work there, as big changes in seating will collapse your powder results. So you split differences back & forth, chasing your tail,, or you accept whatever you ended up with.
2. Back away from any intended charge to do full seating testing. Then go to powder/incremental load development with a ladder and/or OCW and that best rough seating. Pick a powder node, and tweak seating for tightest group shape. The results are either good enough with chosen components, or they aren't.

There are other things about seating you need to mind. For one, MV could go up or down with deeper seating, it depends. If you FL size necks then deeper seating could cause MV to go up. Way up. If you partial length size necks, then deeper seating could cause MV to go down. This, dependent on your CBTO/land relationship. Then you have annealed/hardened condition of necks, adjusting tension, affecting MV slightly & barrel timing more.

Keep in mind that MV & barrel timing are not in direct relationship. And that there are multiple timings affecting results.
You have at least:
-Powder burn timing, affecting muzzle pressures, and SD, and overall forgiveness.
-Barrel whip timing, affecting accuracy at different distances.
-Barrel vibration timing, affecting bore diameter on bullet release.
-Shooting system timing, affecting where the gun is pointed by the time a bullet is released.
So a tune with one powder at this & that distance & rest can produce considerably different MV than tune with another powder.
But all the while, best rough seating holds right where it is, and that's why seating should be handled separately.

Is there a charge/velocity that will also yield better results than any other charge/velocity?

Yes, for your gun, of course.

Given that velocity varies with seating depth, i.e. decreasing with deeper seating, can one find better accuracy/ES/SD by slight decreases or increases in powder charge with seating depth being constant?

Yes, and no. I don't ever give up accuracy for SD, or for 'forgiving'. I go for best accuracy even if leading to razor edge tolerances, and rely on my best in reloading, and planned/limited to validated shooting.
Also, I would not think that seating depth would be the right choice for a powder tuning attribute(like SD).

What would your parameters/methods be for choosing a powder charge after you find the seating depth indicated?

I use QuickLoad 'what ifs' in choosing fastest powder that fills the case by MyMax pressure(which I also tested for during second round of fire forming). And I bounce the powder choice off posted loads at Reloader'sNest, and forums. I load incrementally for ladder shooting at 300yds.
Once I have best hot precision at 300, and it's good enough, I move to cold bore accuracy testing/tweaking at 200yds.