Bumping shoulders

300 ultra

Well-Known Member
Apr 17, 2010
So I have only been reloading a few years and always Full length size my brass. I have read about how I am probably over sizing it and I need a gauge to set up my dies so I'm only bumping the shoulder 1-2 thousands. So I bought the hornady set that fits in my comparator and measured my fired brass then set up my dies like I always due so shell holder fits flush to dies then I rechecked and sure enough it was a .002 difference. So I try multiple calibers and set up the same way as I already do and each one is perfect. .002 shoulder bump. So if I were to just set up how the die manufactures recommend its a non issue. Is this normal? If yes then whats all the fuss about with shoulder bumping. Is it just busy work?
Thanks mikecr, I was hoping you would chime in. I have read a lot of your posts on reloading and definetly respect your knowledge. So in your opinion is this a fluke?
I shoot a lot and most of it is in competition, the way I check my cases is I remove the firing pin assembly from the bolt and size the case so when I close the bolt I can just fill a little bit of resistance. If the bolt handle drops either you oversized it or haven't push the case hard enough to fill the chamber.
Personally, I would prefer that bumping was never needed but it's not like there is any choice there. Eventually it's needed.
I don't think bumping is difficult with better cartridge designs, normal pressure loads, well matched dies, suitable press, and consistent application of a good lube. But accurate bumping can get difficult quick for reloaders who depart from these conditions.

I ran into an ugly sizing/bumping issue once in 6XC with one of Tubb's 'Ferrari of dies', that was seriously mismatching with my T2K chamber(way over sizing). I had JLC Precision make me a bushing/bump die for it, problem solved.
Oversizing the body can result in inconsistent headspacing, to negative headspacing(opposite of bumping). With this, lower body brass is rolled up into the shoulder, countering any bump action. Further sizing still rolls this forward pushed shoulder brass up into necks, increasing donut, tension, & trim length, while still leaving bad headspacing.
Anything can be taken too far..
Having worn out one 26 caliber and three and a half 30 caliber belted magnum barrels in competition, using full length sizing dies set to bump the fired case shoulder back a couple thousandths has always given long case life and excellent accuracy at long range. Brand new cases have produced the same level of accuracy of no worse than 6/10ths MOA at 1000. Sometimes, I've turned case necks to uniform their thickness, but if new cases have no more than 1/1000th spread in neck wall thickness, they aren't turned.

I've never had a problem with that so-called donut at the back of the neck at the shoulder. If it was there, its effect wasn't enough to cause problems. Maybe it was not an issue as my chamber necks were at SAAMI spec allowing a lot of room around the loaded round's case neck. Never used those tight-neck chambers so enamored by many.

Biggest issue with belted cases I've seen is caused by case headspace (distance between head and belt front) is typically a few to several thousandths less than the chamber headspace of .220 inch or more. That allows a ridge a few thousandths long to form right in front of the belt when a new case is fired. And that ridge prevents consistant case fit at the back end of the chamber when it's normally resized. Folks used to cut the bottom and top off a standard full length sizing die, square its bottom, then set that die up in the press such that a previously full length sized case could be sized again all the way to where the botton of that die sized the ridge in front of the belt back down to virtual new case diameter. Chambering these "double sized" cases was done without accuracy robbing interference. Nowadays, Larry Willis Innovative Collet Die's do the same thing.
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