Some Random Thoughts on Headspace, Headspace Measurement and Gauges, Case Measurement and Bullet Seating
There has been a lot of discussion and expressed misunderstanding about Headspace and its measurement and importance on various web discussion pages.
For the definitive work on headspace and all that goes along with it, check out the writings of P.O. Ackley.
For another view on headspace, read on.
An unfired round of ammunition, of a certain caliber, is constructed to dimensions determined by the manufacturer. All ammunition of that caliber, constructed by that manufacturer, will be identical within that manufacture's production limits. When a round is fired in a rifle, the case will expand, in all dimensions, and fill the chamber, -or-, split open before reaching the chamber constraints. Assuming that the case does not split and is somewhat ductile, as brass is, the case will assume chamber dimensions, less about .001 inch in all dimensions due to springback of the metal. This fired case will have "zero" headspace, for all practical proposes, in that chamber.
The difference in dimensions between the unfired case and the fired case are what is important.
In fact, for rimless cases that are headspaced by the shoulder and the head of the case, only the change in length, from the shoulder to the base, is important. The lengthening of the case at the neck is of no importance, nor is the increase in diameter.
It is the "stretching" of the case lengthwise that is important, and is the main reason for "headspace gauges", and the need to measure headspace.
Ammunition manufacturers and rifle makers have decided that only a certain amount of stretch is allowable, somewhere in the range of 7 to 12 thousandths of an inch, any more than that and you run the risk of destroying the casing.
Headspace gauges are made to compare the actual chamber dimensions with the dimensions set by the manufacturer of the ammunition.
It is very important that the headspace gauge match the AMMUNITION that is to be fired, not the chamber.
Certain people are claiming that special "old style" or "old dimensioned" gauges should be used to evaluate military rifle chambers. This is only correct if the ammunition to be used is constructed to the "old" dimensional specifications. Most ammunition, even that made 30 years ago, is constructed to current specifications, and that means that current specification gauges should be used.
The only safe way to determine if a certain rifle/ammunition combination will be safe is to measure an unfired case and then measure that case after firing. This means that a case-measuring tool is required, such as an RCBS case micrometer. If the case stretches by more then 10 or 12 thousandths of an inch, then the rifle is probably unsafe to fire.
If you are a reloader, and use fired cases from a certain rifle, then you have complete control of headspace for that rifle!!!!! Remember, that once fired in a chamber, the case will have perfect headspace. All that you have to do is resize the case without changing the shoulder-to-base dimension. You don't have to use neck-size dies in order to achieve this, just don't, never, ever screw the re-sizing down against the shell holder, like the manufacturer says. Invest in a case micrometer, and adjust the die downward, re-sizing and measuring, until the micrometer shows that you have shortened the case one-thousandth of an inch, and you are ready to go. I AM NOT TALKING ABOUT CASE TO MOUTH MEASUREMENTS. INVEST IN A CASE MICROMETER AND STOP TRIMMING CASES TO LENGTH. YOU WILL GET BETTER CASE LIFE AND HAVE MORE TIME FOR SHOOTING!!!!!
A case length micrometer (RCBS) is shown below.
On the right is a round of 30-06 ammunition loaded at the Lake City Army Ammunition Plant in 1967 for use in the National Matches. I have coated the bullet and case shoulder with blue paint, the stuff used to lay-out machine work. Never fear, you will not use blue this stuff. Next to the ammo, at the bottom, is a micrometer base unit for the 30-06 design, above it is the case length portion of the micrometer. The long white and black bullet shaped thing is a collapsible dummy bullet, and on the right is a loaded bullet length micrometer piece. With the exception of the loaded ammo, everything comes as a unit from RCBS.
The RCBS micrometer provides two distinctly different measurements, first, case length - measured from the case base to the imaginary line on the case neck, and second, overall bullet length - measured from the case base to the place where the bullet diameter matches the bore diameter.
Above is the loaded 30-06 round in the micrometer. Although the micrometer can be used for loaded ammo, it's most useful for unloaded or fired cases. See that the indicated case length is reading "0". This means that the case, headspace wise, exactly matches the industry standard set for the 30-06 case. The graduations are in 0.001 inches (Thousandths)
Above is the loaded round being measures for base-to-rifling engagement length. The reading is 0.143 inches.
Look at this picture very carefully ! When a case is designed the distance from the base to "somewhere" on the shoulder is specified. IT IS THIS DISTANCE THAT SETS THE HEADSPACE SPECIFICATION ! The micrometer has left a mark (line), visible above the leftmost arrow, and has compared the distance from the case base-to-this line with the case designers specification and found it to have a variation of "0". Remember the "0" reading above ?
NOW, LISTEN UP. If you were to measure cases for headspace length and then fire them and re-measure, you could determine the chamber condition as to headspace. BUT MORE IMPORTANTLY, when resizing fired cases for use in the same rifle you should only re-size to about 0.001 less that the fired length !
Additionally, look above the right arrow. Here is the line left by the micrometer when it measured the distance from the case base to the place on the projectile where the projectile engages the rifling. This distance won't mean much to you unless you have first inserted the collapsible dummy bullet in you rifle chamber, closed the bolt, (which collapses the dummy), and then measured the dummy with the bullet portion of the micrometer.
Most reloaders agree that the loaded and chambered ammo should place the bullet close to the beginning of the rifling, say within 0.020 inches and many times closer. This means that you need to find out where the rifling begins IN YOUR RIFLE and then set your bullet seating die so that the bullets end up in the right place. This can be done with the dummy and micrometer.