Thursday, July 07, 2005

Cartridge Case. Our Gas Tank.

Since I have nothing useful to add about the London bombings I shall merely continue with my online exploration of the cartridge. Once we explore the components we'll take a look at how we can make our own ammunition.

The humble cartridge case is the heart of the art and science of handloading. It is the fuel tank, it's an envelope that holds everything together, it's a gasket to keep the pressure from the powder gases from going in undesired directions and it's a jig to keep all the other components in the right alignment to work together. The non-handloader tends to forget that last, the factory worries about alignment so (s)he doesn't have to. I shall discuss alignment in detail when we get into 'how to' part of this series.

Cartridge cases are classified by their priming system, centerfire and rimfire. Rimfire cases are, once fired, scrap metal, they cannot be reloaded. Their rims are folded with a space between the folds. The priming compound is then dabbed in as a thin paste and they were spun in a sort of centrifuge to get the goo into the rim, allowed to dry and then loaded. The factories may well have figured out a more efficient way to do this since I studied up on it as a boy. It doesn't much matter, even if they have I still can't reprime them, nor can I iron out the dent from the firing pin. One of the biggest makers of handloading equipment, Buzz Huntington, got his start making the equipment to turn those fired .22 cases into bullet jackets. When I was a kid varmint hunters still made their own bullets with them. Huntington called it the Rock Chucker Bullet Swage and the company is called RCBS.

It is the centerfire case that interests me, it's the one I can do something with. Centerfire cases are further classified by the method used in controlling headspace. This is a very important word, headspace, I shall lift the definition directly from the glossary of my Speer Manual #13.

Headspace: the distance from that surface of the barrel or chamber that prevents the cartridge from moving further forward into the chamber, to the face of the breech with the action fully closed and locked. This is the most important dimension governing the safety of the shooter. In handloading the combination of cartridge case and firearm must be considered when talking of headspace. To a handloader, few guns need have excessive headspace, since he can adjust the case to fit the chamber, even though the chamber may have excessive headspace when measured by SAAMI standards.

We'll discuss this more, also when we get into the how to part, right now it's enough to know that there are four main ways that we control the headspace, by the rim of the case, by the shoulder of a bottlenecked case, by a belt near the casehead and, in most autoloading pistol cases, by the mouth of the case. We only have a few thousandths of an inch to play with in this, too little and the cartridge doesn't allow the action to close and lock, too much and the firing pin won't reach the primer. Or worse, it reaches the primer allowing the cartridge to fire, yet enough of the brass is unsupported by the steel of the gun to get a KABOOM! The shooter should keep this in mind, the brass of the cartridge case itself is not strong enough to hold the pressure of a high intensity round if not supported by the steel of the firearm.

The rimmed cartridge is the easiest headspace system to visualize, drop the cartridge into the chamber of a revolver and the front of the rim stops it from falling all the way through. Rimmed cartridges are used in revolvers, single shot and lever action rifles and, of course, all rimfires. The rimmed cartridge case would be the only type needed except for the box magazine. The rimmed cartridges don't work particularly well in them. The length of a rimmed cartridge is not particularly important, allowing us to shoot .38 Special cartridges in a .357, for instance, unless it gets so long that it either won't chamber or, worse, will chamber but is jammed so that the caseneck can't expand to release the bullet normally. This causes pressures to skyrocket and had been the cause of more than a few KABOOMS!

The advent of smokeless powder actually brought us the rimless cartridge. The velocities that smokeless made possible made our modern sharp-pointed bullets useful. Sharp pointed bullets and the tubular magazines of the day weren't (and still aren't) a good mix. So most every military in the world went to box magazines, sharp pointed bullets and rimless cartridges, everybody but the Brits. They insisted on sticking with their rimmed .303 and using the difficult to manufacture and oddly shaped magazine in their Lee-Enfields. Headspace on rimless cases is measured from the head of the case to a point on the approximate middle of the shoulder called the 'datum line. A bottlenecked case is the easiest in which we can adjust for a rifle with out of spec headspace in a process called fireforming. The term rimless is actually a misnomer, there is a rim, there has to be to give the extractor something to grab. The rim simply doesn't stick out past the body of the case.

The belted cartridge is simply an attempt to make a rim that will work in the box magazine. We can blame the belted case on Cordite. Cordite, for some reason that if I ever learned I've forgotten, didn't work particularly well in cases with a fairly sharp shoulder angle. Perhaps it was simply a matter of it being long strands. At any rate the belt worked reasonably well in magazines and fairly well to stop the cartridge where it was supposed to. The only cartridges I'm aware of that need that belt are the original Holland and Holland numbers. The trouble is that the wildcatters of the twenties to fifties seized upon those H&H cases because they had more room for powder than the standard cases of the day. Thus the belt became a symbol for power and we are just lately getting away from them. In a modern, sharp-shouldered case the belt does nothing but add expense. Savvy handloaders ignore the belt and adjust their loading dies to headspace on the shoulder.

Rimless straight-walled (mostly) handgun cases headspace on the casemouth. Instead of a gentle angle like a revolver the chamber has a little 'shelf' that stops the case. The person handloading for the .45 ACP and the 9mm Luger should pay special attention to the case length. As the case expands on firing and is squoze down in resizing it will tend to lengthen. If it lengthens too much it will prevent the slide from going fully into battery. In most autoloader this just means a failure to fire. Beware, though, some can fire. When that happens the butt end of the case is unsupported, the high pressure gas blows through the comparatively weak brass and blows the magazine and grips to pieces. Considering that one is gripping those grips at the time, that's not fun. Fortunately(?) it's not common for autoloading pistol brass to last long enough to stretch to the danger point. Between the ejector grabbing it and tearing up the rim and it getting thrown out in the weeds, we don't usually get that many loads out of those cases.

I won't go into detail about how a cartridge case is formed, a description of the process would add more than I've already written. It's quite a process though. Most people don't realize how many operations it takes. Nor do we realize that through a combination of work hardening and annealing the mouth of the cartridge case is considerably softer and more ductile than the head of the case.

The cartridge case is the most expensive part of the cartridge, some uncommon cases can cost over two dollars each. Other cartridges, not all of which are from the 19th Century are no longer made and the only way to get ammunition for the firearms is to make them, starting with the cartridge case. Often they can be made from another case fairly simply, sometimes it's a very laborious process.

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