Shoulder bumping, essential or not?

[rox]

OK - shoulder bumping.

So, we want to 'bump the shoulder' to shorten it. Is the bottom of your die hard up against the shell-holder on your press? If so, it will be impossible to make the case any shorter. What can we do?

We need to shorten either the dies or skim a bit off the shell-holder.

Just to be clear, are you suggesting the following:

If the cases offer no resistance to bolt closure: do nothing.

If there is resistance to bolt closure:

• Where possible adjust FS or Body die towards case head.
  
  Where the above is not possible, start metalwork as you describe.

Wouldn't the need to start doing metalwork suggest that there *may* be a problem with either the chamber, the die or the shell-holder? (or an incompatibility between the die and shell-holder)?

My current process is as follows:

My fired cases consistently mic to precisely +/- zero on a case headspace micrometer (so the chamber size is presumably roughly datum plus one). When the need arises to FL size I use a Redding body die, and adjust to set the shoulder back between one and two thou, to allow for the initial "expand by two, shrink by one" rule of thumb. Does this sound reasonable? I can't say I've ever got to the point where the bolt closure is as difficult as a hot knife through butter!

..

[The Gun Pimp]

Rox,

We are talking about the odd two or three thou. That can make all the difference in the bolt closing without resistance and it being quite difficult to close the bolt.

Your chamber may be say two or three thou. longer than mine - your dies will push back the shoulder without modification whereas my fired case is too short for the die to do anything to the shoulder. Both chambers will still be 'in tolerance'.

For this reason, I always chamber my own rifles 'long' - so my dies will bump the shoulder without the need for modification but if I were to build a rifle for you, it would have to be 'in spec.' - right on the 'go' gauge otherwise it might fail proof - if it was gauged.

If you run your fired cases through the sizing die and they offer no resistance when you close the bolt (minus spring and pin) the only thing to consider is that you may already be bumping the shoulder - but by how much? You need to know this so that you don't over-bump.

With standard factory rounds, there is of course a SAMMI spec. but for wildcats, it's what you (or at least the gunsmith) chooses to make 'em. The best example is the 6PPC - there must be hundreds of different reamers in use - every benchrest shooter has his own idea for a reamer spec.

Cheers
Vince

[artiglio]

Cheers Vince, that explained it clearly, my method was to place a "paper shim" in the holder to raise the case, works ok but obviously the shim soon breaks down and not overly scientific, was going to shim shimstock but next time I'll use your method.

thanks phil

[The Gun Pimp]

Phil,

Good solution - this could give you an extra couple of thou. - which might be enough. A beer-can shim is useful - usually about four thou.

Cheers
Vince

[Dangermouse]

Can you still get feeler gauges from the car shop? I was told that sacrificing a set of those gives nice accurate steps for projects like this.

DM

[The Gun Pimp]

Dangermouse,

Brilliant!

I've just tried it and with a 308 case and shellholder, I could slip in a 10 thou. feeler - which could be enough to give you sufficient 'bump'.

Why didn't I think of that?

Cheers
Vince

[Dangermouse]

Ah, my contribution to shooting achieved, and at such an early stage in my career...

DM

[ovenpaa]

Ah, my contribution to shooting achieved, and at such an early stage in my career...

DM

Heh! when I started reading this I thought you were going to say at such an early age :lol:

[Dangermouse]

The only down side to my trick is that I have no excuse to drink a can of bear first, so not such a perfect plan.

DM

[dromia]

Conversely.

I use tight chambering with a crush fit to get the boolit centred in the throat, a slightly flared case mouth also gives the same result.

As has been stated partial full length resizing can lengthen the case by pushing the shoulder forward and this can be useful for tightening up any headspace play.

Redding do bump dies for pushing the shoulder back. I have one in .303" British as low power loads will eventually move the shoulder forward even though the .303" headspaces on the rim.

I also have one in 6.5 x 55 as getting proper chamber fit is particularly crucial in Swedish Mausers with there fast twist if you want to drive the boolits fast 2,000fps + with any accuracy.