Case Head Separations
by: Germán A. Salazar
by: Germán A. Salazar
First, we need to understand the basic mechanism that will eventually cause a separation. When you fire a cartridge, the case expands radially to fit the chamber; 55,000 psi has a way of causing a ductile vessel, like a cartridge case, to conform to the more rigid material surrounding it! In short, the case will expand to match the chamber under pressure. As the pressure decreases, the case will spring back from the chamber walls to a certain extent. That springiness is one of the principal reasons that we use brass to make cartridge cases. A material with less springiness (such as steel) can be used for case making, but it is inferior; it often causes hard extraction and it is almost impossible to resize for reloading purposes. You'll note that most steel-cased cartridge cases have a lacquer coating, that is to aid extraction, a feature not required with brass cases since they spring back to create clearance.
Once the brass case has been fired, although it isn't as large as the chamber (due to spring-back) it remains larger than before it was fired. That's why we resize the case, of course. Now, as we resize it, we're reducing the diameter of the case along its entire length, but the molecules aren't going back into the original tight lattice, they can't. Instead, the excess material is forced upward along the taper of the case. You'll notice that longer and more tapered cases grow more with each resizing than shorter, less tapered cases. For instance, a 6BR might only need trimming every ten firings, whereas a .30-06 will need trimming every second firing.
As the case grows lengthwise in the sizing process, it begins to thin and weaken just above the solid head. The first picture shows exactly where it thins, since that's where it split. Now let's think about the rate of case stretching and thinning. Part of what we do in full-length resizing is to push the shoulder back to create some longitudinal clearance in the chamber. While it isn't technically correct, we often refer to this as headspace and we'll stick to that usage of the term for simplicity here. I prefer to set the shoulder back 0.001" to 0.002", creating minimal but sufficient headspace (click here for August 2009 headspace article). That doesn't sound like a lot, but it's plenty and will ensure easy bolt operation. That headspace, however, also gives the case a place to stretch longitudinally on firing, that's why we keep it to a minimum as every thousandth of an inch of additional headspace will accelerate the thinning of the web of the case and bring case head separation along that much sooner.
You might wonder if it might make more sense to simply neck-size the brass and thus avoid all of this stretching, thinning and separating. In my opinion, no. Full length sizing keeps the bolt operating smoothly both on closing and on opening and that's important to me in a match as I don't want to be struggling to close or open the bolt while in position. A second consideration is that hard bolt closing will wipe the grease right off of the bolt's locking lugs and they will begin to gall against their seat. In short order you will have a bolt that's almost impossible to operate and an expensive repair bill from your gunsmith. Full-length resizing makes sense to me from a competitive standpoint as well as from a rifle care standpoint.