Re: Which die should I use?
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What is the logic of a case laying in the bottom of the "punch bowl" (best) verses the case fitting the "punch bowl" well supported on its shoulder with its payload well centered on the bore. (worst) What am I missing?
[/ QUOTE ]You, and perhaps most others, may not know where a loaded round fits the chamber both before and when its fired. They don't rest in the chamber bottom. Here's some info that may help you understand what happens to a new case or full-length sized one that's close to new dimensions when it's loaded and fired.
First, when a cartridge is chambered, the back end of a case (new, fired or resized) near the extractor groove is pressed against the chamber wall by the extractor. Mauser style claw extractors press the case straight to the side, sliding extractors like post '64 Model 70 ones in front of a bolt lug press the case to the top and others up and to the side at about a 45 degree angle. This prevents the back end of the case from resting on the chamber bottom. So the back end of the case is never perfectly centered in the back of the chamber when its loaded. The force of gravity ain't as much as the extractor's force pushing the case up off the chamber bottom.
Second, most folks get best accuracy with the bullet touching the lands when the cartridge is chambered. That centers the bullet in the bore. It doesn't matter where the case shoulder is; it may or may not touch the chamber shoulder. As there's no such thing as a perfectly round chamber or case,, there'll be varying amounts of clearance around the front of the case body whether or not its shoulder's touching the chamber shoulder or not.
Third, if the bullet's seated not to touch the lands when the round's chambered, it will float someplace in the throat until the bolt's closed. When the bolt's closed and it has a plunger style ejector, that ejector will push the cartridge forward until it stops against something. Rimless bottleneck cases will stop with their shoulder against the chamber shoulder. The taper of each makes the front of the cartridge center in the chamber. If the bullet's got minimum runout, it's also well centered with the bore axis. Meanwhile the case neck will have some clearance with the chamber neck. If a Mauser style flat ejector is used, the cartridge will be setting someplace between the bolt face and the chamber shoulder.
Fourth, along comes the firing pin and smacks the primer. That energy drives the cartridge forward and it stops against something. A belted bottleneck case will stop when its belt abuts the chamber headspacing belt and its shoulder won't touch anything. As the belt's quite square with the case axis and there's no pressure ring to interfere with the fit, the belted case will align itself quite well centered in the chamber. A rimless case's shoulder will be driven hard against the chamber shoulder centering it there if it's not already there from ejector pressure; usually enough to set the shoulder back a bit due to the 25 to 30 pound force the firing pin spring has that's not all absorbed by the primer. Either way, the bullet's well centered in the bore.
Fifth, gas from the burning powder starts pushing stuff in all directions; the bullet out the case mouth and the case against the chamber walls and bolt face; front part of the body presses hard against the chamber first then the back part 'cause the front part's thinner brass than the back part. The back part stretches back until the case head stops hard against the bolt face. After the bullet's down the barrel pressure drops and the case springs back a bit to smaller diameters and slightly greater headspace.
What's the difference between the above and a neck-only sized case?
First, when chambered, the neck-sized rimless or belted case may slightly bind between the chamber shoulder and bolt face. Its body may touch the chamber wall at different places as there's no such thing as perfectly round cases and chambers and it's larger than a full-length sized case. A belted case not sized all the way to the belt will have its pressure ring in front of the belt contact the chamber headspace belt edge at the chamber body at some point that's not repeatable from shot to shot.
Second, when the neck-sized case is smacked by the firing pin and it's touching the chamber body anywhere, it'll transfer that shock to wherever it touches the chamber, not just the shoulder on a full-length sized case. Those shock waves will go in random directions for each shot using neck-sized cases while the full-length sized one is mainly at the shoulder.
Third, when fired, the neck-sized case walls not already touching the chamber will contact it a bit before a full-length sized case walls do for the same components and barrel. I don't think this matters as much as the pre-firing contact a neck-sized case has with the chamber walls at different points when the firing pin strikes the primer.
The big difference is there's more chances of different and non-repeatable barrel vibrations with a neck-sized case than a full-length sized one because of how the case fits the chamber. The highest scoring off the shoulder rifle shooters in the world have been properly full-length sizing their cases or using new ones for decades. I'm convinced the reason so many folks prefer neck-sized fired cases is three fold. They don't prep new cases properly. They set the shoulder back too far when full-length sizing or don't size belted cases all the way to the belt. They don't test their handloads properly by shooting less than 15 shots per group and often use the smallest group fired as the load's accuracy number. People who regularly shoot more than 15 shot test groups and use the largest groups to indicate accuracy typically end up with more accurate ammo using full-length sized or new cases.
Regardless of what off the shoulder folks get accuracy wise with full-length sized cases, return to battery blocked barreled actions and complete rifles clamped in machine rests have proved over the years that properly full-length sized cases produce the best accuracy. But any case sizing process will sometimes make very small few-shot groups that set records in benchrest. When one compares the size of 20-shot groups, they will usually be surprised at what properly prepped new or properly full-length sized cases will do.
It's my opinion that cases other than new or properly full-length sized ones touch the chamber at more places at the wrong time during firing. New or proper full-length sized ones present more consistant chamber contact during firing else they wouldn't produce best accuracy for so many folks. I've said this before: the smallest groups of 15 or more shots I know of at any range have all been made with new or properly full-length sized cases. I don't know all the techincal details of why nor does anybody else I know of; I let the test groups speak for themselves.
Most of you readers won't believe me; that's fine. Contact Sierra Bullets and the national champions and record setters of highpower competition who shoot 20-shot strings and ask them why they get best accuracy with new or full-length sized cases. They will not know all the technical details but instead let their scores and groups speak for themselves.