Shoulder bumping, essential or not?

[ovenpaa]

Do you shoulder bump? I bump SAUM shoulders back around a thou and a half because everyone tells me I have to, but my .308 Lap brass is only neck sized and stopped short of the shoulder which means that my .308 brass has not been shoulder bumped for up to 8 firings.
Yes 7mm SAUM is not the same as .308 however it does make me wonder if it is necessary in the great scheme of things.

Anyone?

[sanselm]

Dave,

In your chamber or any custom / tight chamber I would only be inclined to bump when the case tells you to, ie the bolt won't close or is getting stiff to close! Vince etc may disagree but on the original SMACK I was able to just deprime it and stuff a new bullet in to go,

Stuart

[The Gun Pimp]

Dave,

Stuart is absolutely right - you re-size as much as you have to - to close the bolt without effort.

I like the bolt to close like 'the hot knife thorough butter' as this will not disturb the rifle on the bags - essential for benchrest - desirable for F Class Open.

To achieve this, your brass must be sized (bumped) to about 1 to 1.5 thou. shorter than YOUR chamber. If you want to know how to do this, I can explain.

Cheers
Vince

[ovenpaa]

Yes please Vince, a tutorial on bumping back the shoulder would be very good as I am not sure I am doing it correctly right now.

[artiglio]

I'd also appreciate some guidance on this, if you don't mind Vince, I understand the principle and why am not overly confident I've got it right.

all the best phil

[Alpha1]

Yes please more info would be good.

[Dangermouse]

And as long as we are going back to basics, can you explain to me how you measure the shoulder - my problem is that I can not see a single point to measure from, I suspect that there is a second tool required?

DM

[The Gun Pimp]

OK - shoulder bumping.

Chambers and dies tend to vary very slightly (unless your gunsmith makes you a re-size die). Dies are made to be very slightly smaller than the chamber so that when you run a fired case through the die, it will easily go back in the chamber.

As we fire our brass, it tends to work-harden - it gets springy - so even when we have run it through the re-size die, we may still experience difficulty in closing the bolt.

Here's what we do. Firstly, remove the guts from your bolt - i.e. firing pin and spring. Now, take a fired un-sized case and put it in your chamber and slowly close the bolt.

Ideally - in a perfect world, the bolt should exhibit just the slightest contact with the case. More likely, it will offer some - though hopefully not great - resistance. From time to time I see shooters really struggling to close the bolt on a loaded round - they just don't know how to set up their dies!

Next take that fired case out of the chamber, lightly lube it and run it through your sizing die. Try it in your chamber again - more or less effort to close the bolt?

Yes, even though you have run it through the die - it could be harder to close the bolt. Reason? You have squeezed the brass and you could have made it longer!

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. Both these items are hardened so not easy to do. If you have a lathe, a carbide-tipped tool will easily cut through the case-hardened die and allow you to shorten it - by about ten thou. If you don't have a lathe, you can grind down the shellholder with a pillar drill. Here's how:

Buy a quarter-inch diameter 'roofing' bolt from B7Q - the ones with the large diameter dome head with a cross in it. Grind or file a couple of flats on the bolt-head so that it will just fit in the recess in the shell holder - where your case fits - then chuck the bolt thread in your pillar drill so that the top of the shell-holder is facing down. Start the drill and bring the shell-holder down on a flat oil-stone using plenty of oil until you have ground off about 10 thou. This will be accomplished quite quickly - once you have broken through the case-hardening. Don't forget to mike the thickness of the shell-holder before you start so you know how much you have ground off.

Now we are in a position to shorten our case a little more.

Now it's a matter of trial and error - screwing your sizing die down a bit more (remember this is a coarse thread - a thirtieth of a turn will move it two thou. if I remember.)

You are looking to shorten your brass so that the bolt will just close with the lightest 'kiss' on the case. If the bolt-handle drops - you've overdone it - back off the die. Ideally we are looking for no more than 1 to 1.5 thou. shorter - otherwise the case will stretch on firing and if you keep doing that you'll soon have case-head separation.

That's it! No special measuring tools Dangermouse - just trial and error. Now your bolt will close like a knife through butter!

If I've not explained it clearly, let me know please.

Vince

[Dangermouse]

Thanks, explained nicely.

DM

[ovenpaa]

Master Class!

Perfectly explained Vince, and thank you to to Stuart for his input as well.