Tuning by seating depth

T

Travelor

New member
Found this question on 6mmBR.com and am looking for a "definitive" answer:


"I use the OCW method to find optimal powder weight followed up by adjusting seating depths to tighten up the groups. These are loads that starts out in the OCW method seated to mag length and later slightly longer (5-30 thousands) to tighten the groups. Please note, the final rounds are nowhere close to the lands.

I was always working under the assumption that the main effect of changing the seating depth in this type of setup was due to changes in case volume which in turn affected chamber pressure, MV, and finally the timing of when the bullet exit the barrel thus allowing the bullet to avoid exiting when the harmonics was at the crown.

However, the more I think about this, the less sure I am of this explanation. The reason being that with the variation in case volume as it comes from the factory, and I only sort brass by weight and not case volume, what is the chance that the case volume is consistent enough to see slight difference in case volume due to slight seating depth adjustments i.e. 1-5 thousands in seating depths.

The other reason is assuming that even if one sort by case volume, exactly how precise is this method? Seems like it is not likely to be precise or consistent enough to allow one to see the slight volume changes due to slight changes in bullet seating depths?

Would very much like to hear what the experts think."
 
Anyone who tries to give you a definitive answer will be assuming a mantle they do not deserve. Remember it has been suggested that in some loads anyway, the bullet may move from primer ignition alone, before the powder ignites. Not sure I believe this, but what does that do to any "case volume" theory? In any case, you need to look at the entire pressure rise curve.

To get definitive answers would require a controlled study, requiring equipment most of us don't have, and test procedures (perhaps) not yet fully worked out.

Bear in mind that "seating depth" works with other factors, such as neck tension (bullet pull), powder type & charge, primers, & individual barrels -- some barrels shoot best with the bullet "jumped," some "jammed." Same brand of everything except the individual barrel.

What is perhaps most telling for a shooter is "how much variance is available." There are reports of rifles (complete system) where jamming the bullet .005 gives superior results, but moving that .002 in either direction causes an unacceptable loss in accuracy. Can you control things at that level? If yes, go for the .005 jam. If, like me, you'd worry, find a seating depth that gives almost as good accuracy, but allows greater margin. It may be a harder jam, or even a jump.

Still, everything interrelates, and there is no guarantee that say, changing the primer, won't extend the sweet spot -- or remove it -- for a particular seating depth.

In practice, a large part of load development is intuitive. You need to get it done before putting too much wear on the barrel. Some point-blank benchrest shooters will give up on a barrel simply because it does not preform with the components they have & are familiar with -- say two types of bullets, one or two types of powder, etc.
 
Charles E said:
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In practice, a large part of load development is intuitive. You need to get it done before putting too much wear on the barrel. Some point-blank benchrest shooters will give up on a barrel simply because it does not preform with the components they have & are familiar with -- say two types of bullets, one or two types of powder, etc.
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Charles, our friend JS does a lot of 0.002" jam change during a day moreso than powder change. How he knows, I don't know other that thirty some years of shooting and phenomenal eyesight that allows him to note what the last bullet did to the group.
 
Jerry, I've long felt -- with no proof -- that withing a narrow region of jam/jump, say .004 off to .007 in, a change in seating depth is equivalent to a powder change.

Problems with this are the starting point of "just touching" varies from one guy to the next, and that some long-range bullets, esp. the VLD designs -- aren't so forgiving. Lynn, when he's reporting on rifle performance, noted that he'd find say, a very good spot at .005 in, but accuracy would go to hell at .002 in. The short-range guys don't use VLDs of course, and the 7-8 caliber tangent ogive bullets seem easier to work with.

* * *

I'm curious what our friend is using for bullets. Did he find Tooley's old dies perhaps? Is he using an old-style Fowler design, or has he gone with one of the "new" designs?
 
I have some Ulrich little uglys, and they flat out shoot no matter were they are on the jam. Those are the type of bullets and barrels i like. LOL!!! Of course we all know that isnt the norm in this game. I like them bullets very much!! Lee

In one particular barrel that i have, anyway.
 
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Charles E said:
Jerry, I've long felt -- with no proof -- that withing a narrow region of jam/jump, say .004 off to .007 in, a change in seating depth is equivalent to a powder change.

Problems with this are the starting point of "just touching" varies from one guy to the next, and that some long-range bullets, esp. the VLD designs -- aren't so forgiving. Lynn, when he's reporting on rifle performance, noted that he'd find say, a very good spot at .005 in, but accuracy would go to hell at .002 in. The short-range guys don't use VLDs of course, and the 7-8 caliber tangent ogive bullets seem easier to work with.

* * *

I'm curious what our friend is using for bullets. Did he find Tooley's old dies perhaps? Is he using an old-style Fowler design, or has he gone with one of the "new" designs?
Click to expand...
Charles, I'll email you.
 
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