View Full Version : School me on 'Shoulder Bumping'
Starting to load for the 308 and I see references to resizing by bumping the shoulder back a certain amount. School me on this process.. how do you measure and determine the amount of 'bump' needed and does this apply even for loading for an AR10 (semiauto) rifle?
I have always loaded for bolt guns and neck sized cases, just recently started to load for the 223 and now taking on the 308, so I want to learn the proper techniques to maximize my brass life!
Thanks!
Its for FL resizing dies, its "bumping" shoulder back just far enough for round to chamber. The thought behind it, is the case isn't worked as much giving longer brass life. Round isn't to SAAMI spec but your rifle spec, so when expansion from firing happens it expands minimal distance to chamber, and when you FL resize said case, its minimum brass that has to be resized. That's probably the lamest explanation of it ever:haha:. Had 6.8SPCII this helped maximize times case could be reloaded. Someone with more insight should be along shortly..*EDIT- I just used Wilson case die, measured fired case and bumped shoulder back .003 just enough for round to chamber. Takes some playin with FL resizing dies to get perfect. This way I do it, not sure if its the "right way" so...
Yea, I understand the basic idea, but I lack the details.. lol
How do I measure the amount of shoulder set-back? Do I need a headspace gauge? How is it quantified from that?
I am planning on building a rifle using a JP Rifles barrel and I know I can get a headspace gauge from them that uses the same reamer as they use on the barrels, so that is on my list..
Yea, I understand the basic idea, but I lack the details.. lol
How do I measure the amount of shoulder set-back? Do I need a headspace gauge? How is it quantified from that?
I am planning on building a rifle using a JP Rifles barrel and I know I can get a headspace gauge from them that uses the same reamer as they use on the barrels, so that is on my list..Well you can get super expensive tool "right way", or get pair of calipers and sleeve that sits half way on the angle between shoulder and neck,on datum line. Measure length, kiss with resizing die to get shoulder bumped back just far enough to reliably chamber. When I did this with 6.8 used moded piece of rifle brass, (wanna say 30-30) and would measure base of brass to top of brass sleeve and go from there. Exactly what a Hornady headspace gauge does https://www.google.com/search?q=hornady+headspace+gauge&source=lnms&tbm=isch&sa=X&ei=lcxtUvP5Ic7xkQerxIDoAQ&sqi=2&ved=0CAcQ_AUoAQ&biw=1366&bih=650 . The only reason I even mention this country boy method to you is because I remember you knowing your way around shop. Just the way I do it.
Forgot to mention case check method, bought it before I knew about brass trick. Put fired case in check gage and use tang of calipers to measure how far shell sticks out. Bump shoulder back enough to chamber reliably, then you have bullet set up for that rifle.
http://www.midwayusa.com/product/479704/hornady-lock-n-load-headspace-gage-5-bushing-set-with-comparator
Do some research around that tool for starters.
What royal posted above is the greatest thing since sliced bread, the Hornady Cartridge Case Headspace Gauge.
Below in front is the Hornady gauge, in the rear are my retired Wilson case gauges. When you buy the Hornady case gauge you only need one gauge, if you buy the Wilson type gauges you need a gauge for each caliber. A vernier caliper will also be needed and can cost as little as $20.00 or as much as $300.00. Anything in the $20.00 to $50,00 dollar range will work fine when you compare it to eyeballing a Wilson case gauge reference marks.
http://i122.photobucket.com/albums/o254/bigedp51/gauge002_zpsd2792ffa.jpg
Shoulder bump, measure your fired case in the Hornady gauge, then start with a .010 feeler gauge resting on the shell holder and screw the die down down until it contacts the feeler gauge resting on the shell holder. Keep using progressively thinner feeler gauges until the desired shoulder bump is achieved. For a bolt action shoulder bump is .001 to .002 and semiautomatics are .003 to .004 shoulder bump.
When you adjust your resizing dies per the instructions and make hard contact with the shell holder and the press reaches cam over at minimum the resized case will be .002 shorter than minimum headspace or the GO gauge. This ensures that the resized case will fit in any chamber, BUT this can also "over resize" the case and shorten its usable loading life and even cause case head separations. The correct minimum shoulder bump will make your cases last longer.
The .308 cases below were resized with the die making hard contact with the shell holder and the press reaching cam over. Below is only and a example, the rifles headspace was not mention and the total shoulder bump was not stated. Meaning if this had been an old worn milsurp rifle the cases may have failed much earlier.
http://i122.photobucket.com/albums/o254/bigedp51/308fail-1_zps30d387ab.jpg
http://i122.photobucket.com/albums/o254/bigedp51/308fail2-1_zps3ca31f6b.jpg
Below is a animated image of a cartridge being fired, shoulder bump is used to control the head clearance or "air space" between the rear of the case and the bolt face. With minimum shoulder bump you are staying within the elastic limits of the brass and when the chamber pressure drops to zero the brass will spring back to its normal size. With excessive shoulder bump you have too much "air space" between the rear of the case and the bolt face and the brass will stretch and thin in the base web area which leads to case head separations. Its just like your Heath teacher told you in High School, if you over resize your cases you will go blind. :haha:
http://i122.photobucket.com/albums/o254/bigedp51/HeadClearance_zpsf30a3af1.gif
Good stuff, thanks! I see I need another tool for my bench, cause I sure don't wanna go blind!!
Great info in this thread! :cool:
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