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Rifles, Reloading, Optics, Equipment
Reloading
Concentricity:How much ?
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<blockquote data-quote="Bart B" data-source="post: 361691" data-attributes="member: 5302"><p>Tactical Froggy's info on that 6mm BR link pretty much duplicates what I've been doing since the late '60's. I lapped out the necks of a few RCBS .308 Win. standard full length sizing dies to .332" and another thousandth for each successive die; I've got full length dies with neck diameters ranging from .332 to .337 inch now. Depending on the loaded round's neck diameter, I'll use a die whose neck is 2 thousandths smaller that it. I set the die in the press to set the fired case shoulder back no more than 2 thousandths. This prolongs case life and helps ensure the neck is well cenered on the shoulder. No expander ball's used. Cases are deprimed and cleaned before lubing and sizing. I use a 50-50 mix of STP engine treatment and Hoppe's No. 9 bore cleaner for lube. I dribble a bit in a foam lined coffee can on a Thumler's Tumbler and lube 20 to 30 cases at a time. This puts a very thin but consistant film of lube on the cases. Case headspace after full length sizing is most uniform using this lube and method; spread's about 1.5 thousandth. I probably would get better uniformity if I used Redding shell holders that come in 2 thousanths steps in height over .125 inch.</p><p></p><p>Full length sizing should reduce body diameters no more than a couple thousandths. That is good and normal for minimum SAAMI chambers which I use. So the shoulder-body junction is now a bit smaller than the chamber at that point. No interference happens like neck only sized cases have when the case has expanded enough that it's sllight out of round now interferes with the chamber when the case shoulder's prefectly centered in the chamber shoulder.</p><p></p><p>When a rimless bottleneck round's chambered, it bears against the chamber wall at its pressure ring 'cause the extractor pushes it there. Bolts with spring loaded ejectors in the bolt face push the round forward until its shoulder mates with the chamber shoulder; this part is centered in the chamber. It' even more centered when the firing pin smacks the case setting the shoulder back a few thousandths. </p><p></p><p>It doesn't matter how much clearance there is around the neck. My chamber necks are .344 to .345 inch and even with .334 inch diameter loaded round necks, they're well centered. If the neck ain't centered on the shoulder, even tight chamber necks won't help; the neck will still be off center in the chamber.</p><p></p><p>Putting the case through a nylon washer that bears mid point on the shoulder contacting it all the way around its circumference and resting the pressure ring between two ball bearings more or less sets up a datum line between center of the case mid point at the shoulder and center of the case at the pressure ring. The dial indicator is 1/10th inch back from the bullet tip to measure loaded round runout. A sized (or fired) case can be checked the same way with the dial indicator just behind the case mouth to check neck runout Both uses are based on the same datum or reference line. </p><p></p><p>As the shoulder's mid point circumference and pressure ring are the two contact points of the case against the chamber when it's fired, it seems right to me that they should be the basis for checking bullet runout.</p></blockquote><p></p>
[QUOTE="Bart B, post: 361691, member: 5302"] Tactical Froggy's info on that 6mm BR link pretty much duplicates what I've been doing since the late '60's. I lapped out the necks of a few RCBS .308 Win. standard full length sizing dies to .332" and another thousandth for each successive die; I've got full length dies with neck diameters ranging from .332 to .337 inch now. Depending on the loaded round's neck diameter, I'll use a die whose neck is 2 thousandths smaller that it. I set the die in the press to set the fired case shoulder back no more than 2 thousandths. This prolongs case life and helps ensure the neck is well cenered on the shoulder. No expander ball's used. Cases are deprimed and cleaned before lubing and sizing. I use a 50-50 mix of STP engine treatment and Hoppe's No. 9 bore cleaner for lube. I dribble a bit in a foam lined coffee can on a Thumler's Tumbler and lube 20 to 30 cases at a time. This puts a very thin but consistant film of lube on the cases. Case headspace after full length sizing is most uniform using this lube and method; spread's about 1.5 thousandth. I probably would get better uniformity if I used Redding shell holders that come in 2 thousanths steps in height over .125 inch. Full length sizing should reduce body diameters no more than a couple thousandths. That is good and normal for minimum SAAMI chambers which I use. So the shoulder-body junction is now a bit smaller than the chamber at that point. No interference happens like neck only sized cases have when the case has expanded enough that it's sllight out of round now interferes with the chamber when the case shoulder's prefectly centered in the chamber shoulder. When a rimless bottleneck round's chambered, it bears against the chamber wall at its pressure ring 'cause the extractor pushes it there. Bolts with spring loaded ejectors in the bolt face push the round forward until its shoulder mates with the chamber shoulder; this part is centered in the chamber. It' even more centered when the firing pin smacks the case setting the shoulder back a few thousandths. It doesn't matter how much clearance there is around the neck. My chamber necks are .344 to .345 inch and even with .334 inch diameter loaded round necks, they're well centered. If the neck ain't centered on the shoulder, even tight chamber necks won't help; the neck will still be off center in the chamber. Putting the case through a nylon washer that bears mid point on the shoulder contacting it all the way around its circumference and resting the pressure ring between two ball bearings more or less sets up a datum line between center of the case mid point at the shoulder and center of the case at the pressure ring. The dial indicator is 1/10th inch back from the bullet tip to measure loaded round runout. A sized (or fired) case can be checked the same way with the dial indicator just behind the case mouth to check neck runout Both uses are based on the same datum or reference line. As the shoulder's mid point circumference and pressure ring are the two contact points of the case against the chamber when it's fired, it seems right to me that they should be the basis for checking bullet runout. [/QUOTE]
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