If you've been reloading for a while, you've probably experienced some case head separations. If you haven't yet, you probably will sooner or later. At a recent match, I had fired four shots when I had a case head separation. When the case head separated, the forward portion of the case remained stuck in the chamber. A quick inspection of the remaining ammunition in the box showed that I would have more separations if I continued shooting it, so there was no sense in trying to remove the forward portion of the case at the range. When I got home, I put a .30 brush on a cleaning rod, ran it into the chamber and as soon as I felt it enter the case mouth, I pulled it back and the front half of the case came with it.
That's a situation we can all avoid, it's no fun, it can be dangerous and it is easy to avoid. While I felt pretty foolish for missing the signs of incipient separation when loading that set of cases, I think that it's all good information to pass along. In this article we'll look at the causes of case head separations, how to anticipate them, how to take care of the problem and how to minimize the occurrence of separations.
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.
Our next picture (above) shows a partial separation. This case split at the web where it thinned from the stretching process, but the split didn't cover 360 degrees, so it held together and extracted normally. The fact that it extracted doesn't mean it wasn't a dangerous situation, however. Gas leaking through the split is still very hot and very high pressure. I'm glad that I was shooting an Eliseo Tubegun which kept all of the gas contained well below my face and eyes.
A frequently expressed misconception about case head separations is that they result from excessive pressure. That is not correct. Pressure, of course, overcomes the strength of the brass case and causes the separation, but that will not happen unless and until the case has thinned excessively through the mechanism described above. Repeated cycles of case sizing with the resultant growth and thinning create the condition which allows normal pressure levels to separate the case at the thin spot. A new case would only separate in a rifle with grossly excessive headspace and one is not likely to encounter a professionally gunsmithed match rifle with that condition.
The third picture shows a sectioned case so that you can see how the case becomes thinner at the web. I've scribed a little arrow into the soot inside the case pointing to the thinned area. I'm not too artistic and the picture shows the thinning better than I expected, so you can ignore the attempted arrow. This case hadn't split yet, but it most likely would have with one more firing. If I had a mill this case section would have looked really artistic, but since all I have to do it with is a hacksaw...
The fourth picture shows where the splits will occur on the outside of the case. The pointer is showing the shiny line that develops on the outside of the case where it is thinning. Look carefully and you'll see some similar looking lines further up the case, these are irrelevant, they're simply rub marks from the ammo box that I use to carry the brass. Down at the web is where you need to pay attention to the case. You should be looking for this kind of bright line every time you reload the set of brass. Most of the reason I felt foolish when I had the separation was that I obviously missed seeing this. I loaded the ammo in a bit of a hurry the night before the match, always a bad idea, and just missed or wasn't looking for case thinning. In part, I suspect it was because the cases had only been fired seven times and I usually get 12 firings from Winchester .30-06 brass before I see incipient splits developing. That's no excuse, however, it was there to be seen and it is a significant safety issue. Whether a case is on its first, fifth, tenth or whatever reloading, you really need to keep an eye out for case stretching marks.
Here's a simple tool that every good reloader keeps in his kit - a bent paper clip. This simple little tool will let you check the inside of cases before you reload them. The thin spot will be immediately apparent as you run the clip up the inside of the case. If you're seeing a shiny line on the outside and the clip is really hitting a thin spot inside, it's time to retire the case. If you do this every time you reload, on at least 15% of your cases, you'll develop a good feel for what the thin spot feels like and how it gets worse as the case is reloaded more times. And if you're loading the night before a match and feel pressured for time - don't skip this step!
The Rifleman's Journal: Reloading: Case Head Separations
