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RCBS Precision Mic
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<blockquote data-quote="Bart B" data-source="post: 369086" data-attributes="member: 5302"><p>I think it's a good idea to mention how a new rimless bottleneck case fits the chamber when it's loaded and fired. This should help folks understand what happens to the case dimensionally.</p><p></p><p>When the round's chambered, it is pressed as far forward as it can go by an in-line ejector in the bolt face such as Remington and the newer Winchester 70's have. The case shoulder centers in the chamber shoulder; this aligns the case neck and the bullet it holds to the rifling. It doesn't matter how much clearance there is between the chamber neck and case neck. If the case neck's not centered on the case shoulder, it won't be centered in the bore. Ejectors external to the bolt such as the Mauser or pre-'64 Winchester 70's don't put any force on the loaded round, so it just sits someplace at its front end. But the case head is a short distance from the bolt face.</p><p></p><p>At the back end, the spring-loaded extractor pushes the case against the chamber typically at the pressure ring (about 2/10ths inch in front of the case head). Depending on the difference in case and chamber diameters at this point, the back end of the case will be some small amount off center in the chamber. The case does not rest at the bottom of the chamber as is so often stated in media print.</p><p></p><p>When fired, the firing pin strikes the primer driving the case hard into the chamber shoulder. If the front of the case wasn't well centered in the chamber before the firing pin struck, it certainly is now. That force sets the case sholder back a thousandth or two (sometimes more) and the case head is now a bit further away from the bolt face. At this time the primer's detonation starts burning the powder. As pressure builds up the bullet gets pushed out and the thinnest part of the case body just behind the shoulder expands against the chamber. As pressure increases, more of the case body presses against the chamber and the back end of the case stretches back. This case expansion shortens the case neck. At peak pressure, the case head has stopped against the bolt face and the outside of the case is pressed hard against the chamber walls. When pressure drops and the bullet exits, the case shrinks back a thousandth or so from the chamber. It's now a thousandth or two shorter in case length than when new.</p><p></p><p>In resizing this fired case to reload it, most folks want the loaded round to fit the chamber good enough to shoot accurately. As long as the sized case ends up with its neck well centered on its shoulder, that will center the bullet well with the bore. But getting the neck centered on the shoulder seems to be the issue.</p><p></p><p>When the case is neck only sized, there's nothing to keep it centered on the case shoulder. If a fired case is neck only sized a few times, its diameter at the shoulder will soon start to interfere with the chamber. This happens as neither case or shoudler's perfectly round and at some point, depending on how the case is indexed in the chamber, interference will happen. The case has to be sized to prevent this interference. If the case headspace is greater than the chambers, the bolt will bind when its closed on such a case and it won't be at the same place for each shot; a known cause of accuracy problems. Expecially when the bolt face isn't squared with the chamber axis. Accuracy is the reduction of all varialbles to zero or as close to zero as possible.</p><p></p><p>A recent poll on one of the benchrest shooting forums showed most of them are moving away from neck only sizing and full length sizing for best accuracy. The don't reduce case diameters nor set case shoulders back much, but they do get consistantly great accuracy that way.</p><p></p><p>Full length sizing that reduces fired case diameters and sets its shoulder back no more than 2 thousandths guarangtees there'll be no interference. The case is free to center perfectly up front and the back end will be a tiny bit off center. How much this makes the bullet crooked to the chamber is less than 1 thousandth of bullet runout would cause. But it doesn't matter as every case is aligned the same amount and direction for each shot; that's consistant. Redding and RCBS make full length sizing dies that use bushings that can be had about 2 thousandths smaller than loaded round neck diameter. These dies don't use expander balls; those darned things that bend case necks and make them off center on the shoulder. One should decap then clean cases before lubing and sizing them in these dies. Sierra Bullets uses the Redding ones for all cases they're available for when tesing their bullets for accuracy. I doubt anyone shoots those bullets as accurate as they do. And they're fired in standard SAAMI dimensioned chambers, too.</p><p></p><p>RCBS Precision Mic's and Hornady LNL case gage (formerly Stony Point) are great. There were a few custom make gages made by a few folks years ago. I got my first one back in the 1960's. After lapping the neck in a few RCBS .308 Win. full length sizing dies for different neck wall thicknesses, I could get 30 or more reloads on cases using maximum powder charges.</p></blockquote><p></p>
[QUOTE="Bart B, post: 369086, member: 5302"] I think it's a good idea to mention how a new rimless bottleneck case fits the chamber when it's loaded and fired. This should help folks understand what happens to the case dimensionally. When the round's chambered, it is pressed as far forward as it can go by an in-line ejector in the bolt face such as Remington and the newer Winchester 70's have. The case shoulder centers in the chamber shoulder; this aligns the case neck and the bullet it holds to the rifling. It doesn't matter how much clearance there is between the chamber neck and case neck. If the case neck's not centered on the case shoulder, it won't be centered in the bore. Ejectors external to the bolt such as the Mauser or pre-'64 Winchester 70's don't put any force on the loaded round, so it just sits someplace at its front end. But the case head is a short distance from the bolt face. At the back end, the spring-loaded extractor pushes the case against the chamber typically at the pressure ring (about 2/10ths inch in front of the case head). Depending on the difference in case and chamber diameters at this point, the back end of the case will be some small amount off center in the chamber. The case does not rest at the bottom of the chamber as is so often stated in media print. When fired, the firing pin strikes the primer driving the case hard into the chamber shoulder. If the front of the case wasn't well centered in the chamber before the firing pin struck, it certainly is now. That force sets the case sholder back a thousandth or two (sometimes more) and the case head is now a bit further away from the bolt face. At this time the primer's detonation starts burning the powder. As pressure builds up the bullet gets pushed out and the thinnest part of the case body just behind the shoulder expands against the chamber. As pressure increases, more of the case body presses against the chamber and the back end of the case stretches back. This case expansion shortens the case neck. At peak pressure, the case head has stopped against the bolt face and the outside of the case is pressed hard against the chamber walls. When pressure drops and the bullet exits, the case shrinks back a thousandth or so from the chamber. It's now a thousandth or two shorter in case length than when new. In resizing this fired case to reload it, most folks want the loaded round to fit the chamber good enough to shoot accurately. As long as the sized case ends up with its neck well centered on its shoulder, that will center the bullet well with the bore. But getting the neck centered on the shoulder seems to be the issue. When the case is neck only sized, there's nothing to keep it centered on the case shoulder. If a fired case is neck only sized a few times, its diameter at the shoulder will soon start to interfere with the chamber. This happens as neither case or shoudler's perfectly round and at some point, depending on how the case is indexed in the chamber, interference will happen. The case has to be sized to prevent this interference. If the case headspace is greater than the chambers, the bolt will bind when its closed on such a case and it won't be at the same place for each shot; a known cause of accuracy problems. Expecially when the bolt face isn't squared with the chamber axis. Accuracy is the reduction of all varialbles to zero or as close to zero as possible. A recent poll on one of the benchrest shooting forums showed most of them are moving away from neck only sizing and full length sizing for best accuracy. The don't reduce case diameters nor set case shoulders back much, but they do get consistantly great accuracy that way. Full length sizing that reduces fired case diameters and sets its shoulder back no more than 2 thousandths guarangtees there'll be no interference. The case is free to center perfectly up front and the back end will be a tiny bit off center. How much this makes the bullet crooked to the chamber is less than 1 thousandth of bullet runout would cause. But it doesn't matter as every case is aligned the same amount and direction for each shot; that's consistant. Redding and RCBS make full length sizing dies that use bushings that can be had about 2 thousandths smaller than loaded round neck diameter. These dies don't use expander balls; those darned things that bend case necks and make them off center on the shoulder. One should decap then clean cases before lubing and sizing them in these dies. Sierra Bullets uses the Redding ones for all cases they're available for when tesing their bullets for accuracy. I doubt anyone shoots those bullets as accurate as they do. And they're fired in standard SAAMI dimensioned chambers, too. RCBS Precision Mic's and Hornady LNL case gage (formerly Stony Point) are great. There were a few custom make gages made by a few folks years ago. I got my first one back in the 1960's. After lapping the neck in a few RCBS .308 Win. full length sizing dies for different neck wall thicknesses, I could get 30 or more reloads on cases using maximum powder charges. [/QUOTE]
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