bumping the shoulder?

[Bill56]

OK, for a person new to reloading, what does this term mean? Then, I've often seen it followed by something like .002". Thanks.

Bill

[MagnumManiac]

The term "bumping the shoulder" refers to partial full length sizing, this can be accomplished a number of ways, with body dies or FL sizing dies, or a combination of both of these and a neck bushing die.

This sizing technique differs from FL sizing by only bumping the shoulder the minimum amount to allow a fired case to fit the chamber with minimum headspace, that is what the .002" refers to, the head to shoulder length. A FL sizing the traditional way will bring the shoulder back to minimum SAAMI dimensions, which may be .006"-.008" or more from chamber dimension, this sizes the brass too much and can lead to short case life.
A neck die is the other method of sizing, but due to the brass expanding after each shot, it will eventually need to be either FL sized or partial FL sized so it will chamber easily again. The partial FL sizing is the preferred method if you want to extend case life and in most instances, accuracy too.

I suggest you buy yourself a loading manual that sets out the different methods clearly and simply so that you can decide which method suite you best.

Cheers.

[Bill56]

Thanks MM.

[Mikecr]

You can bump with a Redding 'body die' as well(they're cheap).
Headspace amounts to the distance a chambered casehead could be from the boltface when driven forward by primer strike. On ignition, the case expands and grabs/seals against the chamber walls beginning at the case mouths and working it's way back to the webs. With this, the case is stretching back to the boltface, slamming against it.
This leads to thinning of case brass back near the webs and in extremes casehead separations(it rips right off, leaving most of the case stuck in the chamber). Also, extremes(like SAAMI + high pressure loads), is bad for pretty much anything you can think of. There is just no good in it.

By far most cartridge designs, with accurate loads, cause cases to stretch a tiny amount with each cycle, eventually causing difficult closing bolt turn. By the fifth reload without shoulder sizing, you will probably be at a point of needing bumps. From this point on, that brass will need bumps(~2thou).

When you FL size brass that stretches a lot on firing back to SAAMI, you are overworking brass, leading to constant trimming. A lot of reloaders do this, I would not. It makes no sense to me to go this route.