Seating depth adjustment

[Jody Simcox]

So far, I’ve just loaded to my desired CBTO...

now did you have to adjust your seating die for each bullet to get your desired CBTO?

Yessir... sometimes as much as.008”.... usually just 1or 2thou

 

[Richard Coody]

Yessir... sometimes as much as.008”.... usually just 1or 2thou

you should either sort the bullets you are using or pick another bullet. the ones you are using have an inconsistent ogive.

try some bergers

 

[Ned Ludd]

snip...So far, I’ve just loaded to my desired CBTO... knowing that some bullets may be pushed down further into the case than others, though.... right?? Makes me doubt my technique (and ability) that’s for sure!

Within reason, having small differences in the amount of bullet shank seated down in the neck are of far less concern to me than where the ogive is located relative to the lands. Think about how far you can change bullet seating depth (which changes effective case volume by changing how far the shank is seated down in the neck) without measurably affecting velocity. It's likely a much larger number than your bullet BTO length variance. So you're doing the right thing IMO by keeping CBTO constant. As far as having to reset your seating die mic, bullet sorting should go a long way toward solving that issue. I think you've got it, but perhaps the cartoon below might help you visualize the critical bullet contact and measurement points.

 

[JimSC]

Within reason, having small differences in the amount of bullet shank seated down in the neck are of far less concern to me than where the ogive is located relative to the lands. Think about how far you can change bullet seating depth (which changes effective case volume by changing how far the shank is seated down in the neck) without measurably affecting velocity. It's likely a much larger number than your bullet BTO length variance. So you're doing the right thing IMO by keeping CBTO constant. As far as having to reset your seating die mic, bullet sorting should go a long way toward solving that issue. I think you've got it, but perhaps the cartoon below might help you visualize the critical bullet contact and measurement points.

by my calculations moving a .30 cal bullet down a full .1 in seating depth decreases case volume by 4.6 ml , a .264 would be 3.6 ml, and a .243 only 3.1 ml so the difference of a couple of thousandths would make is pretty small in the big scheme

great drawing btw

 

[Ned Ludd]

by my calculations moving a .30 cal bullet down a full .1 in seating depth decreases case volume by 4.6 ml , a .264 would be 3.6 ml, and a .243 only 3.1 ml so the difference of a couple of thousandths would make is pretty small in the big scheme

great drawing btw

Yes. In my hands I can generally change seating depth by about .015" in either direction (off the lands) without detectably altering velocity due to changes in internal volume. There are certainly other considerations with regard to non-uniform bullet BTO length and the amount of shank in the neck, but I've always found seating depth (i.e. relationship of bullet ogive to the lands) to be far more critical to precision in most cases than minor variance in the amount of shank in the neck. That doesn't mean having uniform bullet BTO isn't important, but in the OP's case, it sounds like he is currently having to choose between one or the other. I'd personally stick with loading to consistent CBTO for the time being, and continue to work on solving the variance in bullet BTO/shank depth by length sorting bullets.

 

[Jody Simcox]

Thank you, Ned and Jim. Y’all have really helped me out and given me an answer that makes sense to me (and the one I was hoping for). Thanks again!