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Rifles, Reloading, Optics, Equipment
Reloading
Shoulder bumping technique
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<blockquote data-quote="BountyHunter" data-source="post: 1861191" data-attributes="member: 12"><p>To make it clear lets look at the 6.5 creedmoor to help understand the issue. SAAMI specs give a +- of .010 on the shoulder specs for this cartridge. Others can be as low as .007. In other words IF your chamber is absolutely within SAAMI dimension there can be a .010 shoulder variance from one gun to the other. Ammo has to be minimal SAAMI at max dimensions so it will fit ANY gun. Add a worn die reamer and you could add another .002 variance. Did the mftr or gunsmith run the chambering reamer in .002 too long? That is why "basic" die instructions are written for the novice and are set up for cam over to get back to factory SAAMI specs. Their lawyers make them set the instructions up that way. That also means just doing cam over may work and may not by itself.</p><p></p><p>Now let's say you take a sizing die and set it down on the shellholder (again you must assume that they are all the correct height but I have seen .008 variance). I have an RCBS rockchucker and I do not care if you are Magilla Gorilla once metal meets metal that is all the compression and shoulder bump you are going to get. Period!!!!! Now IF you are lucky and IF you can actually measure shoulder bump you might be within .002 of the chamber shoulder. You could also in actuality be about .008 or more off. Again, think about it, that is why Redding has the competition shellholders that offer .0010 of adjustments in .002 increments for the guys hung up on shoulder bump or has dies that are a shade too long for correct bump. Keep reloading that same case time after time at that distance and you will eventually get case head separation. Think about this, how is it even possible to get case head separation IF cam over works perfect every time?</p><p></p><p>There is a reason the top two factory die mftrs (Whidden and Redding) give specific instructions for adjusting the dies for optimal shoulder bump and max case life to back off 1/3-1/2 turn and work your way down. On most dies 1/8 turn is about .009 of actual movement on the shoulder. The new Whidden dies have a .001 click adjustment built in for that reason.</p><p></p><p>Whidden die instructions are to back off and come down. Here is a composite of the Redding die instructions saying same thing for max precision. IF we bother to read the instructions they actually give you two techniques for sizing; one for general use in ANY firearm and one for precision.</p><p></p><p>"Just" getting cam over is for returning back to factory specs and the novice. Period</p><p></p><p>I have had shellholders that were .008 variance from the .125. That is why one shellholder goes in each die box and stays with that set of dies ONLY.</p><p></p><p>Skip Otto die shims are cheap and your friend especially if one set of dies and two guns. Try them.</p></blockquote><p></p>
[QUOTE="BountyHunter, post: 1861191, member: 12"] To make it clear lets look at the 6.5 creedmoor to help understand the issue. SAAMI specs give a +- of .010 on the shoulder specs for this cartridge. Others can be as low as .007. In other words IF your chamber is absolutely within SAAMI dimension there can be a .010 shoulder variance from one gun to the other. Ammo has to be minimal SAAMI at max dimensions so it will fit ANY gun. Add a worn die reamer and you could add another .002 variance. Did the mftr or gunsmith run the chambering reamer in .002 too long? That is why "basic" die instructions are written for the novice and are set up for cam over to get back to factory SAAMI specs. Their lawyers make them set the instructions up that way. That also means just doing cam over may work and may not by itself. Now let's say you take a sizing die and set it down on the shellholder (again you must assume that they are all the correct height but I have seen .008 variance). I have an RCBS rockchucker and I do not care if you are Magilla Gorilla once metal meets metal that is all the compression and shoulder bump you are going to get. Period!!!!! Now IF you are lucky and IF you can actually measure shoulder bump you might be within .002 of the chamber shoulder. You could also in actuality be about .008 or more off. Again, think about it, that is why Redding has the competition shellholders that offer .0010 of adjustments in .002 increments for the guys hung up on shoulder bump or has dies that are a shade too long for correct bump. Keep reloading that same case time after time at that distance and you will eventually get case head separation. Think about this, how is it even possible to get case head separation IF cam over works perfect every time? There is a reason the top two factory die mftrs (Whidden and Redding) give specific instructions for adjusting the dies for optimal shoulder bump and max case life to back off 1/3-1/2 turn and work your way down. On most dies 1/8 turn is about .009 of actual movement on the shoulder. The new Whidden dies have a .001 click adjustment built in for that reason. Whidden die instructions are to back off and come down. Here is a composite of the Redding die instructions saying same thing for max precision. IF we bother to read the instructions they actually give you two techniques for sizing; one for general use in ANY firearm and one for precision. "Just" getting cam over is for returning back to factory specs and the novice. Period I have had shellholders that were .008 variance from the .125. That is why one shellholder goes in each die box and stays with that set of dies ONLY. Skip Otto die shims are cheap and your friend especially if one set of dies and two guns. Try them. [/QUOTE]
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