USSR
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After reading so many posts on the internet, ranging from new reloaders who ask why their newly resized cartridges won’t fit in their chambers, to experienced reloaders who are operating without anything more than a rudimentary knowledge of what they are doing, I felt I should write this article. So, here is a primer as to how headspace works with rimless, nonmagnum, bottleneck cartridges, as commonly used in bolt action rifles. First, essentially, headspace is a relationship between a rifles chamber dimensions (boltface to datum line), and the external dimensions (case base to datum line) of a cartridge case. The datum line is defined as the midpoint on a cartridge’s shoulder and the corresponding area in a rifle chamber. Sure, from a technical viewpoint, headspace is strictly defined as a chamber dimension, and with factory ammo using the standard external case dimensions needed to conform to a SAAMI spec chamber, it is. However, once we enter into the world of reloading, with the many different resizing die setups used by reloaders (brought about by the vague resizing die setup instructions by the die manufacturers), the standard external cartridge dimensions common to factory ammo are usually not to be found.
For the purpose of showing the relationship between a chamber and a cartridge’s external dimensions, I will use the .308 Winchester as an example (see pic.). Headspace standards for commercial cartridges are established by SAAMI (Sporting Arms & Manufacturers’ Institute). For the .308 Winchester, the minimum chamber headspace is set at 1.630”, and the maximum at 1.638”. Most factory .308 Win ammo has case headspace dimensions of 1.628”, which allows for chambering in rifles which have minimum chamber specs. That’s right, I said “case headspace”. Both knowledgeable reloaders and the reloading companies know it as case headspace (Hornady sells a tool called a Headspace Gauge for measuring your brass). Remember, we said earlier that headspace is a relationship between a chamber and the external dimensions of a cartridge case. So, we can see that the relationship is one in which the external case dimensions (case base to datum line) are ideally one in which there is just a few thousands difference between the case dimensions, and the slightly larger chamber dimensions. If your rifle’s headspace measures between 1.630” and 1.633” (you can measure it with a set of Go, NoGo, and Field headspace gauges), you are in good shape. However, if your headspace is on the long side (1.634” to 1.638”), you will likely find yourself with short brass life and perhaps sub par accuracy. From this we can see that by using factory ammo in a rifle with an in spec SAAMI chamber , we will ideally have no more .005” headspace difference between our chamber and case, and certainly no more than .010” difference. So, what happens when we screw our FL sizing die into the press according to the crude die companies instructions? Well, without measuring the case headspace with a tool such as the RCBS Precision Mic (pic.) or the Hornady Loc-N-Load Headspace Gauge Tool (pic.), we have no idea as to the dimensions of our case, and this can and will throw off the relationship between the chamber headspace and the case headspace. Sure, if the newly resized case doesn’t fit in your chamber (insufficient headspace – the most common problem of new reloaders), you can adjust the die further down into the press, but what if your resized case fits in your long chamber (chamber headspace of 1.636”) and your case headspace is now 1.624”? Well, you have just induced an excessive headspace condition in your chamber (even though by SAAMI standards, your rifle does not have excessive headspace), and open yourself up to the possibility of having your brass case come apart from 60,000 psi of pressure, right in front of your face. The .012” difference between your case headspace of 1.624” and your chamber headspace of 1.636”, is no different than the .012” difference in a rifle that fails a field reject headspace gauge test (1.640”) and factory ammo (1.628”). This is why headspace is more that just a static chamber dimension when you are reloading rifle ammo. Hope this helps.
Don
For the purpose of showing the relationship between a chamber and a cartridge’s external dimensions, I will use the .308 Winchester as an example (see pic.). Headspace standards for commercial cartridges are established by SAAMI (Sporting Arms & Manufacturers’ Institute). For the .308 Winchester, the minimum chamber headspace is set at 1.630”, and the maximum at 1.638”. Most factory .308 Win ammo has case headspace dimensions of 1.628”, which allows for chambering in rifles which have minimum chamber specs. That’s right, I said “case headspace”. Both knowledgeable reloaders and the reloading companies know it as case headspace (Hornady sells a tool called a Headspace Gauge for measuring your brass). Remember, we said earlier that headspace is a relationship between a chamber and the external dimensions of a cartridge case. So, we can see that the relationship is one in which the external case dimensions (case base to datum line) are ideally one in which there is just a few thousands difference between the case dimensions, and the slightly larger chamber dimensions. If your rifle’s headspace measures between 1.630” and 1.633” (you can measure it with a set of Go, NoGo, and Field headspace gauges), you are in good shape. However, if your headspace is on the long side (1.634” to 1.638”), you will likely find yourself with short brass life and perhaps sub par accuracy. From this we can see that by using factory ammo in a rifle with an in spec SAAMI chamber , we will ideally have no more .005” headspace difference between our chamber and case, and certainly no more than .010” difference. So, what happens when we screw our FL sizing die into the press according to the crude die companies instructions? Well, without measuring the case headspace with a tool such as the RCBS Precision Mic (pic.) or the Hornady Loc-N-Load Headspace Gauge Tool (pic.), we have no idea as to the dimensions of our case, and this can and will throw off the relationship between the chamber headspace and the case headspace. Sure, if the newly resized case doesn’t fit in your chamber (insufficient headspace – the most common problem of new reloaders), you can adjust the die further down into the press, but what if your resized case fits in your long chamber (chamber headspace of 1.636”) and your case headspace is now 1.624”? Well, you have just induced an excessive headspace condition in your chamber (even though by SAAMI standards, your rifle does not have excessive headspace), and open yourself up to the possibility of having your brass case come apart from 60,000 psi of pressure, right in front of your face. The .012” difference between your case headspace of 1.624” and your chamber headspace of 1.636”, is no different than the .012” difference in a rifle that fails a field reject headspace gauge test (1.640”) and factory ammo (1.628”). This is why headspace is more that just a static chamber dimension when you are reloading rifle ammo. Hope this helps.
Don
