Bumping the shoulder question.....

[chet]

Standard practice on my bench is to set shoulders back .002".
While initially setting up the die, often times I can actually make the case grow... this seems odd to me.
I start with the die quite high, then screw it down and watch the case neck get sized in several steps.
When the neck is sized all the way to where the shoulder starts, I start measuring, this is where the case typically grows. sometimes as much as .006".
I'm assuming that the expander ball is stretching the shoulder upon retraction.... Is this correct?
The die actually has to move quite a bit before reaching my desired measurement.
Funny thing is, it doesn't happen on my 308, and seems to be worse on my 7mm mag. Both are nosler brass.
Just wondering if this is normal? or is there something i am over looking? quality of the brass?

thanks!

 

[DrMike]

You are seeing the effect of the ball dragging over the neck which pulls the brass slightly. You may have a slightly rough expander ball. You can circumvent this by using either RCBS X-FL sizer dies or Lee collet dies. Alternatively, make certain you have lubricated the neck well. I like to use the Imperial Application Media, or a generous helping of motor mica because I like dry media on the neck. Another option if to use Hornady One Shot or through judicious application of any good lubricant to the inside of the neck.

 

[rick smith]

You don't need to move the shoulder back until the case starts to get snug to bolt closure and 0.001-0.002" is plenty. You need to measure to the datum line on the shoulder to know for sure that the shoulder is being moved and by how much. Every time you FL or PFL a case it will grow in length some more than others and yes the expander ball can move the shoulder forward. I try to avoid using expander ball when possible. If I do have to use them, they are polished to a mirror finish and I use a dry lube in the case neck.
Belted mag cases headspace on the belt for the first firing and should be headspaced on the shoulder after the first firing. Having the shoulder move forward is normal. Doesn't move as much with a case that headspaces on the shoulder for the first firing as the 308.Rick.

 

[Guest]

Another reason the case body length grows is that you're squeezing down the case body, and like anything, that "squeezing" causes the object being squeezed to elongate. So when you size the case body, it gets smaller in diameter, but lengthens at the same time. I guess it's basic conservation of matter and the relative incompressibility of solids. Maybe that's a geekier answer than you'd have preferred, but that's the situation. Your 308 is probably a "tighter" chamber and the brass is simply not being resized as much, and thus not elongating. And the 7mmRemMag is probably the most variable chamber in terms of shoulder length of any case I've ever loaded for. I've seen significant variation in the length of 7mmRemMag shoulders across a variety of rifles. The variation is great enough that it's visible to the naked eye, not just visible in the caliper measurements. I think this is why the 7mmRemMag has such a variation on velocities across different rifles, too, but that's another subject entirely.

The discussion about sizing and headspace is spot on, by the way - set all your cases to headspace on the shoulder, regardless of belts or rims, and you'll get longer case life.

 

[chet]

great info guys!!! I understand the happenings much better now!
I'll check the condition of the balls and pay particular attention to lube on the next set.