"Bumping" the Shoulder Back

[jason75979]

I've heard numerous times here and another forum of "bumping" the shoulder back.
I'm guessing this is accomplished by screwing the sizing die in a little more than the recommended setting?
If so, does it work for neck sizing dies as well as FL sizing dies?
How do you determine the correct amount of "bump"? COL gauge?
What are the advantages and the drawbacks?

 

[mehavey]

"bumping" the shoulder back.
...guessing this is accomplished by screwing the sizing die in a little more than the recommended setting?
If so, does it work for neck sizing dies as well as FL sizing dies?
How do you determine the correct amount of "bump"? COL gauge?
What are the advantages and the drawbacks?

"Bumping" is just a slang word for resizing any amount other than
"full length"

How much the shoulder should be "bumped" is determined by
either a headspace gauge, or trial & error.

The objective of either method is to resize just enough to have
the bolt close without appreciable resistance.

Bolt actions guns can use the trial & error method by which the resizing
die is screwed in until the bolt will close manually, and then locked.

Gas guns need a bit more finesse to absolutely, positively ensure the bolt
can close without any resistance at all.

 

[jason75979]

Can you do this with Neck sizing dies?
What are the benefits?

 

[mehavey]

Neck sizing dies do not bump the shoulder at all. They only squeeze the neck
down and should only be used for bolt guns.

The objective of "bumping" (or neck sizing) is to leave as little case "rattle-room" as
possible in the chamber after the bolt closes. This minimizes case stretch upon firing,
and maximizes case life before risk of head separation because of brass thinning due
to that repeated stretching (often dramatically)

See also http://www.youtube.com/watch?v=YE0A5IsR1dA

 

[jason75979]

Neck sizing dies do not bump the shoulder at all. They only squeeze the neck
down and should only be used for bolt guns.

The objective of "bumping" (or neck sizing) is to leave as little case "rattle-room" as
possible in the chamber after the bolt closes. This minimizes case stretch upon firing,
and maximizes case life before risk of head separation because of brass thinning due
to that repeated stretching (often dramatically)

See http://www.youtube.com/watch?v=YE0A5IsR1dA

Okay, I under stand now. It's not so much an accuracy step as it is a life expectantcy precaution.

 

[Jim243]

How do you determine the correct amount of "bump"? COL gauge?

If this is for a bolt action rifle, you can do it with just your rifle and test the resized cases in it (just the case, no bullet, no primer and no powder).

If it is for a semi-auto rifle then you should do "full length resizing" and in some cases "small base full length resizing". There are two types of gauges that can be used, but this is the one I like for setting up my dies.

Jim

 

[mehavey]

It also tends to promote better grouping since cartridge/bullet/chamber/throat alignment is more repeatable with those tighter tolerances.

 

[jepp2]

The goal is to move the shoulder back .002" for bolt action rifles. To give you an idea of how small the die movement is to accomplish this:

 

[Nnobby45]

I've heard numerous times here and another forum of "bumping" the shoulder back.
I'm guessing this is accomplished by screwing the sizing die in a little more than the recommended setting?
If so, does it work for neck sizing dies as well as FL sizing dies?
How do you determine the correct amount of "bump"? COL gauge?
What are the advantages and the drawbacks?

The most difficult part for a newbie on a thread like this is sorting out the useful advice from well meaning, but not so useful advice.

Bumping is a sizing technique that's used for cases fired IN YOUR RIFLE ONLY. You're simply setting the shoulder back just enough for easy chambering in your rifle. The reason for doing it is because once the case is fired in your rifle, it's almost a perfect fit the way it is. But to facilitate easy chambering you size the body and just barely "bump" the shoulder enough to, as I've mentioned, chamber easily in your gun.

NO, you don't screw the die down more. You back it out so it sets the shoulder back less. In the process, the body of the case will also get sized enough. DON'T forget to lube. If you're going to be a handloader, you might as well get a stuck case remover kit now---you'll need it sooner or later.

Smoke the neck of the case, or use a magic marker. Back the die out and then run the LUBED case up and see where the sizing action stops. Keep adjusting the die down until you see the sizing stop right where the shoulder meets the neck. That might do it, but try chambering a case to make sure. DO NOT FORCE THE BOLT DOWN if it starts to stick. Tap the bolt back up and extract the case and size (bump) it a little more.

It's a good technique that keeps the headspace at a minimum and preserves the life of your cases. The case is tailor made for your rifle chamber. And yours only. If may chamber in a rifle with a larger chamber, and stick when chambered in a smaller chamber.

The FL die can be used as a neck sizer. Only half the neck needs to be sized to hold the bullet securely. You'd still have to lube the case and it offers no advantage over FL sizing. I define FL sizing as any amount of sizing in a FL sizing die that sets the shoulder back to any degree.

Someactually think full length sizing is running the case up so far there's a lot of cam over (bump). They size the case more than is necessary to chamber in their rifle because they read the instructions for GENERAL sizing and don't size for their specific rifle.

For cases not fired in your rifle, you just have to size them to whatever degree is necessary so they'll chamber in your gun. Don't size them any more than that.

NOTE: When you size a case in the FL sizer, the case actually grows longer in the critical shoulder area until the shoulder contacts the mated portion of the die and sets it back enough (makes it smaller again) to chamber. That's why the case will rechamber after you've fired it, but not chamber after you've improperly sized it by not sizing enough.

 

[Ideal Tool]

"Neck sizing dies...should only be used for bolt guns".
I used to feel bad about being a late starter on internet...Now I feel very fortunate in not having it when I was starting out..so much heresy and half-truths.
I have been neck sizing..with RCBS, and now using the Redding bushing type dies for my single shots for over 20 years.

 

[mehavey]

I've heard what you said
Now hear what should have been heard:

"...They should not be used on autoloaders...."

If you would like to expand further on "...heresy and half-truths...," please feel free to do so.

 

[jason75979]

I've heard what you said
Now hear what should have been heard:

"...They should not be used on autoloaders...."

If you would like to expand further on "...heresy and half-truths...," please feel free to do so.

You're speaking of the neck sizing dies, correct? This is what I have read in every book I own. FL size for auto loaders.
Thank you all for the info.

 

[mehavey]

You're speaking of the neck sizing dies, correct? This is what I have read in every book I own. FL size for auto loaders.

Correctamundo.....










postscript: You "can" actually do a shoulder bump on gas guns, but it's advanced citizenship because the ejection process with residual chamber opressure can/does blow the shoulder forward a few/several thoundandths.

There is therefore no such thing as "chamber" forming unless you cut off auto-ejection entirely. The other challenge w/ gas guns is that you've got to strip the bolt of extractor/ejector before you can really get a reading on whether the bolt closes freely on a case. Then you need a Headspace gauge (either Hornady's or an RCBS precision Mic) to measure exactly what that "freely-closing" headspace case dimension really is so you can repeat it without fail in the future.

So I tend to FL-resize as a matter of course, and toss the cases after a half-dozen loadings (or so).

 

[Elkins45]

^^^ My gas guns have a built in feature where they automatically discard a certain percentage of my cases for me, whether I'm finished using them or not.

 

[mehavey]

My gas guns have a built in feature where they automatically discard a certain
percentage of my cases for me, whether I'm finished using them or not.

I call those 'Grass' guns.




.

 

[603Country]

Go buy the Nosler handloading book and read the section on "Partial resizing". It will explain, with diagrams, how to do what you want, which is a partial resize (using a standard resizing die) that sets the shoulder back just enough to chamber the round easily though snugly. Read that and you should come away with a good understanding of what is happening and how to do it. It isn't complicated at all and Nosler explains it well. And like the guys said, this will chamber size the cartridge case for YOUR rifle and only your rifle, so don't go swapping the partial resized ammo between rifles of the same caliber.

My Nosler book, which is not the newest one, has that information. I will assume (right or wrong) that they have that info in the newest version.

 

[F. Guffey]

603County,

Bump, still, after all these years, bump sounds like an accident. I have presses that bump, all my presses that that bump, bump twice, and I have 3 Rockchuckers, not one of the three is a bump press, jam, cram, lock over or lock up but they do not bump.

I believe Nosler and I could carry on a civil conversation, most of my dies are ‘Versatile dies’ I form/size cases from short chamber length from .012 thousands shorter than a minimum length/full length sized in thousands to .016 thousands longer than a a minimum length case. That covers full length sizing to minimum length to go-gage length to no go-gage length to field reject length and every length beyond and before, that includes 28 different length. For the same money I get neck sizing, partial case body sizing neck sizing with case body support.

I did not say I did not have other dies, as to other dies, my favorite dies are case forming/trim dies, if I had one form/trim die it would be a 308 W.

I guess bump and accident go together when making a wild guestimate of an adjustment, I eliminate accidents, I use the companion tool to the press, the feeler gage when adjusting the die to the shell holder, with all of the different methods and techniques available, to me it makes no sense to use ‘a method’ without verifying, and if the reloader is going to verify why go to the effort with other methods and techniques and then use the feeler gage to verify, why not go straight to the feeler gage (standard) to verify and skip the wild guestimate.

Verifying, transfers, standards and feeler gages do not lock me up.

F. Guffey

 

[jason75979]

Okay, I'm still new to this so please don't complicate it too much.
Mr. Guffey, I understand "bump" is basically a slang term.
I also realize if using this method there should be a "checks and balances" step . Whether it be feeler gauges or calipers. I actually only wanted to understand the terminology, not, start an argument.
So please gentlemen, info is extremely appreciated, arguments only lead to confusion, clouded actions, and or mistakes for "newbies" such as my self.

 

[603Country]

Jason, again let me say that this partial resizing is not rocket science. It's a very simple concept and easy to do, particularly once you understand the objective and the process. Get the Nosler book and read and absorb the section on Partial Resizing. All you are basically going to do is to take a fired case and resize it...but just a teeny bit less than fully resizing it. That'll leave the case formed to fit your rifle chamber and it will resize the neck by about 75% or 80% of its length. You'll want to experiment with just the right amount of resizing, so that when you chamber a round it'll be a snug fit but not so snug that you have to really force the bolt handle down. PM me if you want to discuss this on the phone - but you still want the Nosler book.

 

[jepp2]

603Country, could you list what edition of the Nosler manual you have?

I have the current version - #6 - and their advice on sizing die setup is marginal at best, dangerous at worst. For full sizing they give the standard 1/4 turn past full contact for all dies. And for partial sizing they say to leave 1/16" gap between the shell holder and the die. So what you find most useful didn't make the new manual.