Bumping headspace?

[elwaine]

I don’t quite understand the concept of “bumping headspace.” When should one employ that technique rather than fully re-sizing the case.

If I start with once-fired factory brass (fired from my rifle) I know I can simply re-size the neck using my neck sizing die with bushing in place. After subsequent firings, I can continue neck sizing only, until chambering gets a little tight. (Actually, I measure the headspace using a Wilson headspace gauge rather than waiting for the chambering to get tight.) At that point:

1) I can re-size the entire case, including the case neck, using the full-length sizer die with the neck bushing inserted; or,

2) I can use the full-length sizer die simply to set back the shoulder a tad (“bumping the headspace”). - BTW, I assume that when one bumps the headspace, the neck bushing is not used… is that correct?

When the headspace increases, why would I only want to set back the shoulder and not completely re-size the case? And if I chose to employ the technique of bumping the headspace, when will I have to fully re-size that case rather than just continuing to set back its shoulder?

 

[USSR]

elwaine,

I also have the Redding necksizing die with bushings. Rather than use a FL sizing die to bump back the shoulder, I use the Redding body die which does than and minimally resizes the body as well. This die does not touch the neck, so you will need to necksize the case as you normally do. Question: have you removed the expander ball from your necksizing die?

Don

 

[moosehunt]

Actually, I've never heard that term. You basically have it correct, though--neck size until they get too tight, then FL size once and go back to neck sizing. Generally, when they start getting tight, both the shoulder needs to be moved back a skosh, plus the body needs a bit of reduction. You will most certainly need to trim after the FL sizing. I'm not very familiar with the specialized Redding dies, and there may be some other approaches with them.

 

[elwaine]

USSR,

Actually, I have the RCBS Gold Match dies: neck sizer, full length sizer, and the bullet seating die. Like the Reddings, the sizing dies utilize neck bushings which vary in size by .001" increments. When used with the neck bushings, the expander ball is replaced with a more narrow device to hold the de-capping pin.

I have nearly 2,000 fired factory cases that I saved up before starting to re-load, so I have never used new, unfired brass. The expander ball might be needed in brand new cases - but I'm not sure.

 

[Coltdriver]

This notion of bumping the headspace is something I have just learned about.

I have a Ruger #1 and it is notorious for vertical stringing. I recently learned that this is a usual symptom of lacking headspace in a falling block.

The Encore and Contender have the exact same problem. The nature of the action is that the case stretches.

So it is possible to full length size then trim the neck and still have a shoulder to base measurement that eliminates all headspace.

I thought that with a single shot I was much better off to fire form to the chamber and then neck size only. But with the Encore this is a bad idea exactly because of the headspace going away.

I picked up a cheap dial micrometer at tool king and was really surprised to see my case heads extending past the end of the chamber 4 to 6 thousands on my Ruger. It was less on my .223 but enough to eliminate the headspace.

So the process I followed to get my .243 AI cases dialed in this evening was to de cap. Full Length size. Then remove the expander ball from the decapper and bump the cases back a few thou. I am thinking about running them back through the full length one more time. Bit of a pain to go through all of that but I can't wait to go try some of these new loads out.

 

[Bullet]

If your going to check headspace I recommend the RCBS Precision Mic.

A few twists of the Precision Mic and you'll know your chamber headspace and bullet seating depth to 0.001". The Precision Mic measures from a datum point on the case shoulder to the base, giving you the exact SAAMI tolerance readings. An indispensable tool for safe, accurate loads that extend the life of your brass and firearm.

http://www.midwayusa.com/esearch.ex...o+Begin+Search.x=7&Click+to+Begin+Search.y=11

elwaine
I don’t quite understand the concept of “bumping headspace.” When should one employ that technique rather than fully re-sizing the case.

You might want to just bump the shoulder when you don’t want to over work the brass (size it more than necessary). Fully resizing the case is not needed in many rifles.


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[elwaine]

You might want to just bump the shoulder when you don’t want to over work the brass (size it more than necessary). Fully resizing the case is not needed in many rifles.

Ahh... that's the answer I was searching for. Thank you Bullet! So, I suppose it's a trial and error sort of process. Instead of fully re-sizing the case when the headspace increases too much, you simply bump the headspace back a bit and see if that works in your rifle - and if not, then resort to full re-sizing.

I'll give it a try. Thanks again...

 

[USSR]

elwaine,

If you are using a FL die to "bump the shoulder back", you cannot do it without fully resizing the case at the same time. By the time the shoulder is far enough up in the die to bump the shoulder, the entire case has been resized as well.

Don

 

[elwaine]

If you are using a FL die to "bump the shoulder back", you cannot do it without fully resizing the case at the same time. By the time the shoulder is far enough up in the die to bump the shoulder, the entire case has been resized as well.

Thanks Don, but that is not correct... at least not according to the instructions that came with my die set.

These are the instructions for bumping the headspace: "To set the Gold Medal Full Length Sizer Die to "bump" the headspace, with the shell holder in the ram and the ram at its highest point, screw the Sizer die down until it contacts the shell holder. Back the die up 1/2 turn and secure with the lock ring. Take a properly lubricated case that no longer chambered in the firearm and size it in the the die. Remove the lubricant and check to see if it will now chamber. If it does not, relubricate the case and adjust the die body down in 1/8 turn increments until it does chamber properly in your firearm." [Neck bushing removed from die.]

These are the instructions for full case re-sizing: "Screw the Gold Medal Full Length Sizer Die into the reloading press until the sizer die touches the standard shell holder with the ram at the top of the stroke. Lower the ram and set the die 1/8 to 1/4 turn further down so the press cames over the center. Set the large lock ring and tighten the set screw with a set screw wrench." [Neck bushing in place within the die.]

So you can see that in full length re-sizing, the die is set below the contact point with the shell holder (ensuring full compression of the case into the die), whereas in bumping the headspace, the die is set above the shell holder contact point so that only some of the case enters the full length re-sizing die (setting back the shoulder a tad, without fully re-sizing the whole case).

For me, learning to reload - even after extensive reading, some personal instruction by a seasoned re-loader, and with good fellows like all of you guys are who are willing to answer my questions - is like walking through a fog to get to a better place. I really do appreciate all of your comments and assistance. - Larry

 

[Walkalong]

It is still sizing the case, unlike a special die cut to neck size with a bushing and just touch the body so when you bump the shoulder back the body can't move outward. It is not sizing as much as it can, but it is still sizing the case body.

 

[Sunray]

Cartridges do not have headspace. Headspace is a firearm manufacturing tolerance only.

 

[USSR]

Thanks Don, but that is not correct... at least not according to the instructions that came with my die set.

Sorry, elwaine, but it sure as hell is correct. The shoulder being pushed back is the very last thing being done when the brass case is fully into the die. How the hell do you not resize the body, when the body is all the way into the die? Sometimes ya just gotta use common sense.

Don

 

[Bullet]

elwaine
I suppose it's a trial and error sort of process

Not if you use a precision mic. See my post above.

Walkalong
It is not sizing as much as it can, but it is still sizing the case body.

I agree with Walkalong.

 

[USSR]

Cartridges do not have headspace. Headspace is a firearm manufacturing tolerance only.

Sunray,

You continue to post this, knowing full well that excessive headspace or not enough headspace can be induced by the degree that the shoulder is bumped back. What is called "case headspace" is known and understood (or should be) by reloaders of bottleneck cartridges.

Don

 

[Coltdriver]

Headspace is the measurement between the breech face and the base of the cartridge.

Manufacturers establish the headspace based on a sammi spec cartridge and chamber.

When you are reloading used brass YOU establish the headspace by changing the relationship between the base of the case and the shoulder of the case by "bumping" the shoulder back (for a bottle necked cartridge).

When you reload, you are the manufacture of the ammo and you can no longer rely on a preset range for headspace established by the manufacturer of the rifle.

On another note I wish there were one of those RCBS Precision Mics that could be used with a .243 AI.

 

[elwaine]

Thanks Don. I understand now. What you said makes sense.

 

[Hikingman]

Headspace and 303 British - Cartridges

There's an interesting article on headspace, visit:
http://www.rifleshootermag.com/gunsmithing/headspace_0612/

In the article, Patrick Sweeney wrote:

On a rimless cartridge such as the .308, the stopping surface is the shoulder. On a rimmed cartridge like the .30-30 or .303 British, the headspace is the gap for the rim...A rifle with excess headspace can eat reloaded brass quickly...Upon firing, the expansion of the case blows the shoulder forward. When you resize and set the shoulder back, you set up the excess headspace condition again. The stretched brass will eventually crack and break. A prime example of this is the .303 British.

 

[USSR]

When you are reloading used brass YOU establish the headspace by changing the relationship between the base of the case and the shoulder of the case by "bumping" the shoulder back (for a bottle necked cartridge).

Zactly!

Don

 

[Bullet]

HEADSPACE
The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.


http://www.saami.org/Glossary/index.cfm


Maybe I’m wrong but I think headspace is a fixed measurement. It’s a measurement of your rifle. When you resize your case you aren’t reducing headspace but rather reducing the distance between the base of the case and the datum point on the case shoulder. A RCBS Mic measures this distance on a fired case and can be used to tell how much you’ve reduced this measurement by resizing. There by allowing you to not size your case more than desired. A Mic is used to accurately set the amount your sizing your cases. As an example when I resize my cases for my AR, I measure a fired case and then adjust my sizing die to resize the case .002 to .003 smaller between the base of the case and the datum point on the case shoulder (and by the way this measurement is different for each of my AR’s). This amount of resizing is enough for reliable chambering with out sizing my brass more than necessary.

I believe when resizing your case your sizing compared to the rifles headspace.


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[USSR]

Maybe I’m wrong but I think headspace is a fixed measurement. It’s a measurement of your rifle. When you resize your case you aren’t reducing headspace but rather reducing the distance between the base of the case and the datum point on the case shoulder. A RCBS Mic measures this distance on a fired case and can be used to tell how much you’ve reduced this measurement by resizing. There by allowing you to not size your case more than desired. A Mic is used to accurately set the amount your sizing your cases. As an example when I resize my cases for my AR, I measure a fired case and then adjust my sizing die to resize the case .002 to .003 smaller between the base of the case and the datum point on the case shoulder (and by the way this measurement is different for each of my AR’s). This amount of resizing is enough for reliable chambering with out sizing my brass more than necessary.

I believe when resizing your case your sizing compared to the rifles headspace.

Yes, headspace is a fixed measurement in a rifle's chamber, based on SAAMI's cartridge dimension spec's. If you have an old rifle with excessive headspace (based on using SAAMI spec factory ammo), well then you've got a condition where the cartridge base to cartridge datum line (mid point in case shoulder) is too short for the chamber, resulting in short case life or case failure. You also have excessive headspace when you have a rifle with perfect headspace dimensions and you bump the shoulder of your brass too far back, also resulting in a condition where the cartridge base to cartridge datum line (mid point in case shoulder) is too short for the chamber. Coltdriver explained it perfectly, but I hope this also sheds some light on it.

Don

 

[Khornet]

Dedicated bump dies

are available. I think from Wilson or Sinclair, or even Midway. But it seems a lot of trouble to go to when you can just FL size once, fire, and resume neck-sizing.

 

[Walkalong]

Headspace in a rifle is a static measurement relative to that rifles chamber in comparison to ammo loaded to SAMMI specs. Both the chamber and the ammo is controlled by SAMMI spec.

That goes out the door with ammo not loaded to SAMMI spec, because headspace is the allowable amount of dead space where the case expands. Now you have the same good chamber that has no headspace problem by SAMMI specs, but you yourself have created headspace by bumping the shoulder back beyond SAMMI spec.

When you create excessive headspace by bumping the shoulder back to far the case has to much room to expand. The front of the case grabs the chamber while the back half stretches until the case head stops at the bolt. If there is to much manmade headspace you can have a case seperation. Even if it does not happen the first time, it will happen soon.

Real headspace can be changed by bumping the shoulder back too far. period.

Don and Coltdriver are 100% right on this one, with no room for debate.

IMHO of course.

 

[Bullet]

I believe this is one of the times that I sometimes find the way things are said or the terms used vary. I couldn’t find anything on case headspace, dead space or real headspace in SAAMI’s glossary but I did find this and believe that this is what your referring to -

From SAAMI -

HEAD CLEARANCE
The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace.

I believe that different terms are alright to use if the meaning is understood.


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[Walkalong]

:banghead:

 

[Bullet]

This link might help -

http://en.wikipedia.org/wiki/Headspace


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