Cartridge head space and shoulder bumping

[rockydoc]

Cartridge head space as I understand it is measured from the face of the cartridge head (face of the bolt) to that part of the cartridge which stops the cartridge from moving any farther into the chamber(the datum). In the case of the .308 family of cartridges the datum is that point on the shoulder which is .400" in diameter.

This is a definition, not an explanation and can be confusing. Isn't head space effectively just the space between the bolt face and cartridge head? If there is too little head space the bolt will not close on the cartridge? In other words the cartridge is too long from the head to the datum. If there is too much cartridge head space the cartridge is too short.

The more dangerous of those two situations it would seem to me, would be too much cartridge head space, when the cartridge is too short. If there is so much space the firing pin could not strike the primer there is not so much danger as would be the case where the firing pin drives the case forward until the shoulder stops it then the primer ignites the powder charge. The pressure expands the case, the neck obturates and anchors the case to the chamber wall while the head is not touching the bolt face (too much head space) allowing the head to separate from the case releasing up to 60,000psi of expanding gases. That could, as they say "cause equipment damage and bodily harm".

Now to the subject of "shoulder bumping". I am not sure what that means. I think it means that if a cartridge shoulder has moved forward and making the cartridge too long (too little head space) you would move the shoulder back in a full length sizing die.

I have .260 Remington cases (new Nosler) which I have never full length sized. I have neck sized only with a Lee Collet Neck Sizer. They have been fired and reloaded three or four times.
The numbers for my cases measured with the Hornady Head Space Gage with the .400" datum:
New unfired and unsized= 1.611"
Fired and reloaded 3-4 times neck sized only= 1.618"

SAAMI chamber specs:
Minimum = 1.630"
Maximum = 1.640"
SAAMI cartridge case specs:
1.6340"~.0070" (Does this mean the allowable amount of space between case head and bolt face is .007"?

If that last line means that a range of 1.6340 to 1.6270 (1.6340-.0070), then my brass is shorter than any of these dimensions. Does this mean that my head space is too great? I have not had any signs of trouble. I would think that if my head space was too great I would have some elongation of cases, wouldn't I? The controlled round feed of my Kimber rifle should hold the case to the face of the bolt, allowing the firing pin to strike the primer, but if the excess space is between the case shoulder and chamber shoulder that would allow cases to elongate. But they haven't.

There is more to this than I can grasp. Or am I making things more complicating than they need be?

Rockydoc

NRA Life member since 1956, Benefactor

 

[918v]

No, factory chambers allow as much as .011" in my experience. Your headspace is fine.

 

[USSR]

Isn't head space effectively just the space between the bolt face and cartridge head? If there is too little head space the bolt will not close on the cartridge? In other words the cartridge is too long from the head to the datum. If there is too much cartridge head space the cartridge is too short.

Headspace is measured by the distance between the boltface and the datum point of the shoulder, as you stated. While the net effect is space between the boltface and cartridge head, the gauges for measuring headspace are not something that measure that space.

Now to the subject of "shoulder bumping". I am not sure what that means. I think it means that if a cartridge shoulder has moved forward and making the cartridge too long (too little head space) you would move the shoulder back in a full length sizing die.

That is correct. You can use either a full length sizing die or a body die to bump the shoulder back, but not a necksizing die, as that die only interacts with the neck.

...am I making things more complicating than they need be?

Yes.

Don

 

[Walkalong]

Assuming your calculations are correct, .007 is more than I would want, but safe enough with brass that is in good shape. For a bolt gun, .001 to .003 is what I like to shoot for for safety and long case life. For a lever gun I like to keep the shoulder where it was after the first firing.

 

[MEHavey]

If you're neck sizing only, your bolt-action rifle has "adjusted" those cases to near perfect headspace fit upon firing. Bolt actions do their best when the headspace is either "perfect" or a thousandth or two under. The theory is that while the bolt can always cam-close on a case a few thousandths too long, it is likely to ever-so-slightly distort the shouder/neck and induce some run-out misalignment of the bullet at the end of that neck -- degrading accuracy.

Repeated firings in even a bolt action will eventually cause the case to grow to where the shoulder exceeds the chamber's headpace dimension. "Bumping" the shoulder back to either that perfect fit (impossible), or to the classic 0.002" under (possible/repeatable) is the objective. The Hornady Headspace Gauge will help you do that with some precision.

In an autoloader, however, the bolt must not meet any resistance to closing or you risk a slam fire when the firing pin starts forward before the bolt is fully closed. There, the case must be resized every time at least that 0.002" (some say 0.004") under chamber dimensions. The Hornady Headspace Gauge will help you there as well.

 

[Walkalong]

http://www.thehighroad.org/showthread.php?t=504759

 

[243winxb]

Cartridge Head Space & Shoulder Bumping

Brass is said to have spring back after firing, how much i do not know. You should neck size till a round will not chamber at all, after the brass work hardens. Then may be you will get a longer cartridge measurement? The Hornady Head Space Gage is a comparator . It measures fired brass, not chambers. As said in this info http://www.brownells.com/.aspx/lid=12897/GunTechdetail/Gauging_Success___Minimum_Headspace_and_Maximum_COL

Hornady makes an inexpensive ($45) cartridge headspace gauge (framed) that can check the headspace reference dimension of: new brass, cartridges and, almost, a gun's headspace dimension.

Almost being the key work. These gauges are used to measure a rifles chamber for the correct headspace.

 

[rockydoc]

Walkalong,
The .007" mentioned is my assumption from the SAAMI case specification of 1.6340"~.007", not from any measurements.

You mentioned you like to have .001" to .002". My fired cases measure 1.618".
How do I achieve the .001"-.002" for my cases in my chamber.

 

[918v]

By adjusting your die so it only bumps the shoulder by .001"-.002".

 

[243winxb]

Cartridge Head Space & Shoulder Bumping.

How do I achieve the .001"-.002" for my cases in my chamber?

Rockydoc, When you full length resize, this is when the shoulder should move forward. But you must FL size by controlling the dies adjustment. Adjust/set the FL die with a .010" thick feeler gauge between the shell holder & FL die. Size, measure head to shoulder. Has it moved forward? If not, adjust the FL die again with a .008" feeler gauge. Then .006", then .004" or until the shoulder moves forward. As the body of the brass is pushed inward during FL sizing, the shoulder/neck move forward. This is why its best to trim brass after FL sizing.

 

[brickeyee]

By adjusting your die so it only bumps the shoulder by .001"-.002".

You can purchase case gauges designed to measure the shoulder dimension, and then adjust the sizing die to move the shoulder back exactly as much as you want.

 

[918v]

or a Redding competition shell holder set.