In response to my comments:
Every RCBS, Pacific and Lyman press I've full length sized cases on setting fired case shoulders back a thousandth or so average. The FL die's set so there's a few thousandths clearance between the shell holder and die's bottom at the top of the ram's stroke.
How much spread (case head to shoulder datum measured with case headspace gauges) is determined on how uniform the case lube's on the cleaned case as well as the amount of spring the press has. The type of case lube also effects the exact amount of shoulder setback in the FL die.
Woods replies:
Odd. Instructions with dies typically say to raise the ram and adjust die down to contact with the shell holder, then lower the ram and adjust down an additional 1/4 turn for full length sizing. Creates "cam over". NO space between shell holder and bottom of die, in fact compression.
Yup; that's what the instructions typically say. The reason is the die company wants reloaders to full length sized fired cases enough so they'll fit easily in the chamber of the rifle. Die companies do not know the dimensions of the fired cases reloaders use. Nor do they know the chamber headspace (bolt fact to chamber shoulder) in the rifle barrel those reloads will be used in. Full length sizing dies for bottleneck cases set in the press to "cam over" when the case is full into it typically have their shoulders set back to about SAAMI minimum case headspace. (Example; SAAMI .308 Win. case headspace = 1.627" minimum, 1.634" maximum.) If one shoots a rimless bottleneck round with too much head clearance (bolt face to case head when fired), incipient head separation is starting and, if excessive, will happen on the first firing.
Leaving space between the shell holder and bottom of the die typically means only part of the neck is sized, the amount depends upon the amount of gap between the shell holder and die. So in full length sizing there is no gap between the shell holder and bottom of the die.
I and most full length sizing die makers consider this a common belief as well as a popular myth. Years ago, I had a long discussion with RCBS about this very thing at a Shot Show in Las Vegas. We both agreed that FL sizing dies have less "die" headspace (bottom of shell holder to die shoulder datum when holder's touching die bottom than SAAMI specs for GO headspace gauges. In order for your comment to be true, every fired case would have to have the same case headspace, all FL die's chambered exactly the same and all barrel chambers having the same headspace. That's not going to happen as there's a large spread in chamber headspace across all the barrels chambered for a given cartridge and a smaller one across all the die's headspace.
For example, of the several .308 Win. FL dies I have, each one set in the press with the ram/shellholder at the top and screwed down just barely snug on a .308 Win. 1.630" GO headspace gauge, FL sizing fired cases from a 1.630" headspaced chamber ends up pushing the shoulder forward a few thousandths past 1.630" case headspace. Cases so sized will not let the bolt close on them when chambered. Proof the fired case shoulder gets pushed forward as the case body gets sized down. I measured case headspace with a gauge custom made back in the '70's long before the Hornady LNL (old Stoney Point) case gauge and RCBS Precision Mic's were available to do this. I use the RCBS Mic now; works great for belted magnums, too.
Backing the die up further results in FL sized case shoulders moving more forward.
You put lube on the shoulder??!!
Yes. Why not?
If the die is locked down on similar cases then the amount of takeup on the press linkages will be the same with each stroke.
I have full length sized 25, 50 or 100 cases at a time, checking the measurement between case head and shoulder datum continuously and have NEVER noticed a "few" thousandths variation, ever. Maybe .001" very occasionally (read RARELY) but never that much variation.
This is normal when the shell holder stops against the die's bottom. More often than not, this usually sets the fired case shoulder back too far for best performance in the chamber it came out of. If the rifle's chamber is on the long end of specs, there'll be way too much head clearance for long case life and best accuracy. And this problem's compounded by how much the case shoulder gets set back by firing pin impact when its fired.
And this is the reason why folks years ago used to face off the bottom of shell holders so their "height" could be made higher and end up sizing fired case shoulders not as far back using holder-to-die contact when sizing. Redding finally made shell holders in .002" height increments to do the same thing. It's long been a practice by folks to full length size bottleneck cases setting the shoulder back 1 to 2 thousandths for best accuracy. But you've got to set the FL die in the press correctly to do it. Using the labels in the following link will help set the die correctly when using a case headspace gauge:
http://i860.photobucket.com/albums/ab170/jepp2/DieAdjustment-1.jpg
Odd, are you really meaning this?
'Tain't odd. It's normal. All it takes to learn exactly what happens is proper use of the right measuring tools on cases and die setup in the press then correctly interpreting the numbers. Many folks have known this for decades.
Meanwhile, I agree with mikecr; all that matters is the seating that produced best accuracy. There can be a few thousandths spread in jump distance to the lands and accuracy will rarely suffer any signifcant amount. Or, soft seat bullets in case necks so they push back a few thousandths as they contact the lands when chambered. This is often the best way to seat bullets for best accuracy; it very well centers them in the bore.
