Headspace is a really important factor while resizing our brass. Resizing can be done in a few different ways however I prefer the full length sizing better than neck sizing. Before going into the process I would like you to understand what actually happens to the case when it is fired so you will better understand what happens with the case headspace.
Factory “non-rimmed” also called “rimless” ammo is designed to have the shoulder stop forward movement of the case. Factory “belted” ammo is designed to let the belt stop any forward movement of the case. Factory “rimmed” ammo is designed to have the rim stop any forward movement of the case.
A new “rimless” (non-rimmed) cartridge fits the chamber rather loosely, the bullet is not in perfect alignment with the bore and the case doesn’t contact the front of the chamber. As the firing pin strikes, it moves the case forward to contact the front of the chamber, giving a little headspace – but not a dangerous amount. The violence of the ignition backs the primer out of the pocket until it’s rearward movement is halted by contact with the bolt face and as the powder is ignited and pressure builds up, the brass case expands to fill the chamber completely, preventing any escape of gas to the rear. As the pressure continues to build, the case is forced so tightly against the chamber wall that it cannot move, but since we had a gap between the base of the cartridge case and the face of bolt – what we termed a little headspace – the case itself must stretch driving the head of the case against the bolt face and lengthening the case. As the case is moved rearward the primer is also reseated in its pocket. Once the bullet exits the muzzle the pressure drops, the case cools, and the brass contracts enough to permit extraction of the fired cartridge case from the chamber.
With the “belted” magnum case actually the process is really similar except that the firing pin doesn’t push the case forward to the point that it would touch the front of the chamber, because as we said before the belt is designed to stop the cartridge from moving forward. All other processes are the same as with the “rimless” cartridge. It is really similar also with a “rimmed” cartridge where the rim prevents the cartridge from moving forward.
With a fired case we have a very close model of the rifles chamber and now it is time to determine headspace. So what is headspace actually? The term headspace means the “space” between the “shoulder” of your case and the bolt face. Regardless how factory loads fit your chamber, handloads should always have minimum shoulder clearance. Most reloaders commonly refer to chamber clearance as “headspace”. However, the term “headspace” actually makes things a bit confusing for some people. For handloading purposes, the term headspace means any chamber clearance that allows your cartridge to move or expand forward or rearward. There’s no need to make things more complicated than that. To determine the headspace of our chamber use a fired case from our rifle and some sort of headspace gauge. There are more companies that produce headspace gauges like Innovative Technologies, Hornady, Sinclair and Whidden. I am sure that I have missed some. It is up to you to decide which one to use. On the photo below you can see a fired .300WM case a digital caliper and Hornady headspace gauge.
I like to full length size my cases for a few reasons. One is because when only neck sizing sooner or later (approximately after 3-4 firings) depending upon the operating pressure, the cases will lengthen to a point chambering will become difficult and must be full length resized again. Second thing because I hand load for hunting and need the cartridges chamber smoothly.
So why do we need to measure headspace and how do we use this measurement to determine how to adjust the sizing die?
With the standard process of resizing we would adjust the resizing die as it is stated in the Die Instruction manual. By doing so, we may actually bump back the shoulder of the case more than is necessary. This may result in premature case failure due to case head separation as the brass is repeatedly stretched and resized. Although this process will work there is a better way for the experienced handloader to extend case life and enjoy knowing the case dimensions are optimal. Since I expect that those reading this article are experienced handloaders and will want to know how to resize their cases in a more detailed way. Now that we have determined our headspace we want to resize our cases with a bump of .002-.003” I suggest .002” for a bolt action rifle and .003” for an autoloader. This will insure reliable operation while at the same time avoid over stretching the cases.
We can use Redding competition shell holders which come in sets with .001” increments that are made for this purpose, or we can use a standard shell holder and adjust our die by screwing it in or out in small increments so that it will bump the shoulder back for .002”.
For this I suggest to deprime a few cases (5+) and measure the headspace of all of them. There will likely be a small variation due to the properties of each case and how the brass springs back. Once the average is established we will want to move the shoulder .002” shorter.
The process with Redding competition shell holders is really simple. We start with the highest shell holder and measure the resized case. We then use the next smaller shell holder and repeat the process until we measured the headspace to be .002” smaller than the headspace before the resizing process.
If we use a standard shell holder we need to adjust by screwing it in or out in small increments while measuring headspace. It is a little bit more difficult and we will probably oversize one or two cases but once set it is the same. Here I suggest to set the die at least a bit away from the shell holder. Then we start resizing the first case.At first we will see only the neck being partly sized. We continue to screw in the die by 1/8 of a turn until we have sized the full neck. Then I suggest only to turn the die in slightly and measure the headspace. We repeat the process until we get the correct amount of bump. Below you can see the case that was resized and the headspace is .002″ smaller.