Please clarify

[Nickwisenheimer]

I finished my first read through of my Lyman manual. I cant seem to understand what head space is. A little help please.

[Waldog]

Headspace is the distance from the breech face to that part of the firearm’s chamber that stops the forward movement of the case.

The following is not absolute. But as a "general rule" the following is valid:
Rimless rifle cases headspace on the shoulder of the case.
Rimmed cases, both rifle and handgun, headspace on the thickness of the rim.
Belted Mag rifle cases headspace on the belt.
MOST semi-auto handgun cases headspace on the case mouth.

[dmazur]

Here's a link to an explanation, with some diagrams -

http://www.surplusrifle.com/shooting...pace/index.asp

A headspace gauge is a convenient way to determine if a rifle has excessive headspace. This compares the dimension described above to a standard maximum acceptable length. The standard is used to make ammunition to a consistent and safe dimension that will be short enough to fit all rifles but not so short as to be dangerous.

After firing thousands of rounds, it is possible for bolt lugs and engagement surfaces to wear and create a longer than acceptable chamber headspace. The most common solution is a new barrel, which is sold "short chambered" and finish reamed by a gunsmith to the standard dimension.

Excessive headspace is dangerous because it can result in case head separations. The body of the case is held by friction in the chamber, but the head is thicker and does not expand to grip, so it is pushed back until it impacts the bolt face. This results in stretching at the head, which can lead to a rupture if it is large enough. With poor brass quality control in WWI times, case head separations weren't all that uncommon and measures like having every other soldier carry a broken case extractor and designing gas vents into the action were used to deal with it.

Case head separations aren't common with modern rifles and modern ammo.

However, an improperly-adjusted resizing die can result in creating cases with a headspace dimension that is shorter than the chamber dimensions, creating excessive headspace even though there is nothing wrong with the rifle.

This is a common misuse of the term, describing case length to datum (typically rimless with shoulder datum) as headspace. What is meant is that the case length is too short, which has the same effect as excessive headspace in the rifle.

Further confusing the issue are cartridge headspace gauges, which are the counterpart to chamber headspace gauges. Where the chamber headspace gauges look like steel cartridges, cartridge headspace gauges resemble a chamber. The user drops a fired or resized case into the gauge and compares the case head position to a couple of steps. If it is even with the top step, you have maximum case length (and theoretically zero headspace), and if it is even with the bottom step, you have minimum case length (and thefore maximum acceptable headspace.) And, of course, we are still talking about case length to datum, not case length to mouth.

IMO, most of the confusion about headspace seems to be related to this dual use of the term. It is used to describe both a chamber dimension and a case dimension. What is important to understand is that both dimensions are important, and the difference between them is what you have to control if you want your resized cases to chamber reliably and have acceptable life.

[wogpotter]

Headspace is, put simply, using the exact same dimension for a critical part of both the case & the chamber it fits in.
Chamber & case work together during firing, the case swells to fit the chamber & the chamber supports the case. Organizations like SAAMI decide on the dimensions & everyone uses those specs so everything interchanges between manufacturers.
If the matching dimension is beyond the tolerance for either they stop working together & the system fails.
Lots more detail in the posts above, but that's the bare-bones basic 'splaination, Lucy!

[Crusty Deary Ol'Coot]

OK Nick,

You have already rec'd some good info as per your question, so I am going to take it a bit of a different direction.

Excessive head space, can be a pain and even dangerious,

But slight head space situations, while possibly not dangerious, can cause very short case life and inconsistancy in your hand loads.

NOTE THE FOLLOWING! --- All fire arms and reloading/sizing dies are, OR SHOULD be made within "manufacturing tolerences".

In a perfect world, everything would be perfect and exact, but in general, the firearms chambers and sizing dies, while within tolerence and safe to use, are not perfectly made.

So, let's say you have a rifle that happens to be on the big side of the allowed specs. and you have a set of reloading dies on the small side of the allowed specs. this will cause a slight amount of excessive head space in your reloaded ammo.

Not enough to be dangerious, but enough to grealy shorten your brass life.

So, the secret is to seldom follow the manufactures instructions for setting up a "full length" sizing die, but rather to set the die down ONLY to the point where your loaded ammo chambers relyably.

Now, If your interested, I'll gladly walk you through this process, but only if you ask. No need to bore you otherwise.

Keep em coming!

Crusty Deary Ol'Coot