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Old November 4, 2013, 09:55 AM   #26
Bart B.
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Not much except to insure that the fire-formed, unsized body of the case keeps the round aligned with the bore instead of dropping to the bottom of the chamber.

A full-length sized case cannot align the bullet with the bore and makes for a sloppy fit with the extractor often effecting the alignment also.
Probably the most stated "myths" in reloading for accuracy.

Those believing this is true, take the bolt out of your rifle, twist the cocking piece so the firing pin falls, then look at the bolt head. There's one, sometimes two things that push the case head a few to several thousandths away from the bolt face such that the shoulder on a rimless bottleneck case mates perfectly in the chamber shoulder. That centers the case shoulder, neck and bullet very well in the bore regardless of how much clearance there is around the case to the chamber wall. There's something else that pushes the back end of the case against the chamber wall; up for some, to the left or right and sometime up a bit, too, on others.

While the extractor does effect alignment, it's effect is microscopic and one needs stuff that shoots sub 1/100th MOA to see its effect. Besides, it's 100% repeatable from shot to shot so its influence is masked by that repeatability

If you cannot figure out why such cases center perfectly in the chamber's front end, post your issues and I'll explain further. This is really easy to figure out; just pay attention to all those things that put pressure on the case head after the bolt's closed. After figuring all this out, you'll also learn why few, if any, rimless bottleneck cases rest on the bottom of the chamber after the bolt's closed and especially when they're fired.

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Furthermore, full-length sizing induces neck run-out.
Only when the fired case neck is sized down way too much and the expander ball comes up through it; this usually bends case necks somewhat. Setting the expander ball to start up the case neck while most of the case neck's in the die helps keep it straight. Using bushing dies without expander balls eliminates this issue.

There are ways to physically measure case neck alignment with the bore on chambered rounds. It's interesting to watch the case neck center up perfectly when the round's chambered, even with brand spankin' new cases in oversize chambers. Especially watching a new .243 Win case neck perfectly center in a .308 Win chamber.
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Last edited by Bart B.; November 4, 2013 at 10:20 AM.
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Old November 4, 2013, 10:53 AM   #27
dahermit
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If you cannot figure out why such cases center perfectly in the chamber's front end, post your issues and I'll explain further. This is really easy to figure out; just pay attention to all those things that put pressure on the case head after the bolt's closed. After figuring all this out, you'll also learn why few, if any, rimless bottleneck cases rest on the bottom of the chamber after the bolt's closed and especially when they're fired.
Explain please, why some cases have off-center firing pin indentations and the pressure "ring" around the head is more pronounced on one side than it is the other.
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Old November 4, 2013, 11:33 AM   #28
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I'll be honest, it's only very rarely that I have seen a case fail because it has spilt at the neck. I have seen signs of pending case head separation mostly in .303 Brit. All the loads I did for my 303 Brit and 7.62x54r I neck sized only and had no issues.

Maybe I haven't reused my brass enough to start seeing failures.

So if neck sizing isn't the cats meow for accuracy....when is it appropriate to neck size verses full length resize? And why is neck sizing toted by all the major reloading companies as something that increases accuracy?
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Old November 4, 2013, 11:57 AM   #29
Bart B.
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Explain please, why some cases have off-center firing pin indentations
Bolt face is off center from the chamber axis, or, the firing pin hole's off center in the bolt face. Sometimes the extractor pushes the case far enough off center on the bolt face to do that. Measure your "stuff" to find out what dimensional and alignment issues there may be.

An easy way is use an empty case with a well-centered flash hole, remove the firing pin and cocking piece from the bolt, then chamber and close the bolt on that case. Look through the back end of the bolt with the barrel pointed at a bright surface. Does the case flash hole align well with the firing pin hole in the bolt face??????

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and the pressure "ring" around the head is more pronounced on one side than it is the other.
Case walls are thinner on one side; that's what expands the most relative to the case head. It can be anyplace around the chamber, wherever the case thinnest walls are near the case head.

Cut the case off a bit in front of the pressure ring then measure their thickness all the way around. Then check the more pronounced bulge relative to the case wall thickness as well as the firing pin's "finger print" and you'll see that bulge is at all clock numbers away from the pin's finger print axis.

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So if neck sizing isn't the cats meow for accuracy....when is it appropriate to neck size verses full length resize?
I cannot think of any appropriate reason to neck size.

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And why is neck sizing toted by all the major reloading companies as something that increases accuracy?
Probably for the same reason most of their customers do. Ask them and find out why. Most folks have a lot of misconceptions on how bottleneck cases headspacing on their shoulders fit the chamber when fired; some of them own, manage and work at reloading die companies.

In contrast to what die companies so often champion, Sierra Bullets has been touting proper full length sizing dies and techniques for best accuracy for decades. They use Redding full bushing or standard full length sizing dies on fired cases used to shoot their bullets testing for accuracy.
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Last edited by Bart B.; November 4, 2013 at 01:13 PM.
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Old November 4, 2013, 01:10 PM   #30
wncchester
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"... it's only very rarely that I have seen a case fail because it has spilt at the neck. I have seen signs of pending case head separation mostly in .303 Brit."

You're setting your case shoulders much too far back, that's what causes excessive stretching and head seperations. Well, that and a 'springy' action, especially one with rear locking lugs.


"All the loads I did for my 303 Brit and 7.62x54r I neck sized only and had no issues."

Neck sizing doesn't set shoulders back at all; that one thing prevents head seperations but it's not that necking, per se, is doing anything special.


"Maybe I haven't reused my brass enough to start seeing failures."

You're seeing a failure it seems, but a worse one. Correct your FL sizing and you'll see neck splits, they are harmless but head seperations can be serious.

I've not seen that "all the major reloading companies" say necking improves accuracy. Some loading manuals have articles written by paid "experts" that say so but, in 48 years of reloading, I've NEVER seen any loading company make that claim; in fact some loading companies specifically say otherwise. If you still think all gun writers are the last word for anything you're in for more disappointments.
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Old November 4, 2013, 02:33 PM   #31
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Correction, i've never had a case head separate. However I have seen where a shiny ring forms around the circumference of the case. Using a pick I could reach down into the case and feel a divot where the case wall was thinner, this corresponded with the ring. I was told when loading for the SMLE actions the cases tended to stretch a lot and too keep an eye out and toss cases....I used them for much lower pressure cast loads.

I'm always up for new info and corrections. If neck sizing is a bunch of bull and I'm wasting my time doing it then I am all for changing things up. In any rate thanks for the explanation and correction, I'll try and pass that along next time someone asks.
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