Bumping

C

CalleT

Guest
Hi ,
New to handloading I wonder why should one "bump" cases?
And how do I "bump" if I have to do that?

Good shooting all

CalleT
 
I'm used to the term "bump" being used to describe how much the shoulder of the case is set back when sizing the case.

Is that what you're asking about?
 
Bumping is the act of moving the shoulder of the case shorter by .001 or .002. As you load by neck sizing only the shoulder of the case progressively has less spring back on each successive shot with that case. Eventually you will no longer be able to chamber the case, then you need to bump the shoulder back and start the process all over.
Bumping must be done with a full length sizing die so that it does not swell the case at the body shoulder junction.
Also, generally when you bump the shoulder you will have to trim the case.
Use good tools to measure shoulder bump, excessive shoulder bump can radically shorten case life.
 
The amount that a case is bumped is just the amount that the shoulder to head measurement is shortened by a sizing operation. Setting bump involves the measurement of a fired case, that is tight, and setting the die so that the measurement that is taken from the shoulder to the head is reduced by a given amount, say .001. There are dies that are designed only to move the shoulder, and dies that size the neck and move the shoulder. Common practice in short range benchrest is to use a FL die to that sizes the entire case, very slightly in the body, and set it so that the shoulder is pushed back, bumped, around .001. To do this accurately, one needs the correct measuring tool (usually a caliper attachment), and to be sure that the primer does not influence the measurement of the fired case (Re-seating the fired primer accomplishes this quickly.).

Some prefer to set dies by feel. I do not recommend this approach because it presumes that the die is small enough to size the base of the case, and also have the bump within an acceptable range. This may not be the case, and if it is not, the shoulder may be set back too far before the desired bolt feel is achieved.
 
Bumping may not be necessary if you crank the powder measure down to a reasonable number of clicks. Anything below the Skip Otto level will probably work. I've told this before, but I once went an entire season with 20 cases and all I ever did was neck size them. About 1500 rounds total so you can figure yourself how many times each one was fired. The only reason i threw them away was the barrel was toast and Skipper read me the riot act and led me by the hand over to the garbage can. True story.

Ray
 
Ray, it was a question for the op.
Without some kind of idea what he is shooting bumping could be a moot point.
I thought since it was the General Discussion area it would be a good time to ask.
 
Vern - good point. The same thing occured to me. A new guy shooting a Winchester 30-30 or a Garand doesn't need his head filled with all the details of a Benchrest shooter's methods and procedures. Hell, I'll go one step further, 90% of shooters do not need that kind of detail. I've noticed, since Al Gore invented the Internet, that shooters everywhere are asking if they should anneal, bump, turn necks, measure ogives, jam, etc, etc. It takes the fun out of shooting, is usually unnecessary, seldom results in improvements that they think will automatically appear.

IMHO - Ray
 
You know Ray, there is a part of me that likes all the stuff we do. Each year when I start turning brass I think about how much I like it, heck I like it so much maybe I should consider offering it as a service to others......then after about case #15 I start having second thoughts and after about case #30 I start wishing I could just preload before going to a match. At case # 80 I wonder just wonder why is it I am doing this again?????
 
Vern, I shoot 6.5x55 Swed AI and 222Rem with necksiezed cases. But I also have 7x57R and 9.3x57 but I don't necksieze them since I do not shoot that much in those calibers.
Thank You all for good answers :)

Good shooting all

CalleT
 
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