Seating Depth Testing

This is an expectation that does not make sense to me- varying pressure with seating depth to refine the tune. Why will a variation in depth cause a beneficial preasure change, when a .1 gr change in charge weight will not? Not to mention that .1gr is usually within the standard error band of weighed charges.

As a corollary, is optimal seating depth the same for a given bullet and cartridge when using different powders? (same barrel of course)
 
Whether it makes sense or not, try changing depths 20 thou at a time - from 20 in to 80 out - then shoot - you will see a remarkable difference!
There are many variable that come into play here, its not just one thing (pressure) - the cumulative effect is that it plays a part in barrel harmonics, and finding the sweet spot shrinks the groups.
Seat depth has a relationship to pressure, bullet to rifling engagement, may help with un-concentric loads, can give more space in the case for more powder, etc,etc... However in saying that, i have a few rifles that seat depth made no difference - an example was a Rem 5R i had - shot best with book length loads and 1/4" of jump.
 
Been there done that, seen the effect- I'm not arguing whether it works- I do it myself because it does have an effect. My question is why does it work.

If its voodoo, that's good, I'll start working on my pre-match rain dance.
 
chrismc said:
i have a few rifles that seat depth made no difference - an example was a Rem 5R i had - shot best with book length loads and 1/4" of jump.
Click to expand...

so far my rem 700p shoots the best at book length and 1/4 inch of jump also... anything longer is full of flyers.

not like I can even get close to lands anyway
 
Did a quick oal measurement on a factory Tikka 6.5SE barrel- I jump 123 Scenars 0.150" in it, and it works. If I would try to seat out to the lands, I'd only have about 100 thou of bearing surface in the neck. Yes, 100 thou.
 
Thanks for all your advice guys. I found that Berger method to be very helpful. I really did not want to load in 5 thou increments from touching to 100thou, that's a lot of wasted bullets. So I'm gonna do 10,40,80,120. Then fine tune from there.
 
max-x said:
Been there done that, seen the effect- I'm not arguing whether it works- I do it myself because it does have an effect. My question is why does it work.

If its voodoo, that's good, I'll start working on my pre-match rain dance.
Click to expand...

Seating depth and powder charge are related... changing either has the same effect on the harmonics of the load. You are changing the parameters of the Kaboom. When this goes out of tune for what the barrel wants, you have stringing. Change back into tune and all is good.

So if you have a very accurate scale and seating die, do this test. Set a OAL anywhere you want off the lands, work up the powder charge to reach the best accuracy possible - hold to under 0.1gr accuracy. Change the OAL say 10 to 30 thou, odds are the tuning will now be off... adjust powder charge again and voila, you end up in the same accuracy.

It has become very popular to set the OAL, adjust powder then adjust OAL as a final step. I believe this stems from so many loading scales having a significant amount of error. The actual amount of powder in the cases are not the same. however, micrometer adjustable seating dies allow for very small changes in OAL which tend to be more reliable and repeatable.

That very small change in OAL "acts" the same as ultra small changes in powder charges... voila, bring the load into tune.... or at least within the window where the error in the powder charge is still within the accuracy node of the barrel.

Anyone want to take a guess on what a 3 to 5 thou change in seating depth acts like wrt to powder charge?

Try it, it is alot of fun and very enlightening. Will you find bullet/powder/gun combos that are more immune to this? Of course but most precision LR rifles will show the results.

Have fun...

Jerry
 
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