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· Sincere as a $5 funeral
1,636 Posts
Discussion Starter · #1 · (Edited)
The Brass Cartridge

NOTE: I'm not really too happy with where I went with this. If you think it comes up short of making any kind of point, I have to agree with you.

I was surprised to find that there is still some controversy over the function of the cartridge case. You know, the brass part.

In the ideal case, where the entire cartridge is closely contained, as in a bolt action or other "strong" fixed action, the brass case serves primarily as a gasket to prevent leakage of combustion gases (from the primer and powder charge) back through the action.

But there are a couple of other things that it has to do in real guns. As always, I will be writing about straight walled pistol cartridges. Since the interesting part of this subject is related to failure, I will omit the general description of cartridge behavior, and skip to the chase.

Discussion of the brass cartridge case generally goes off the rails when the subject of "strength" arises. Arguments based on ultimate pressure and wall thickness are pointless. In fact, all modern brass cases are much stronger than they need to be, and all are nearly equally able to withstand repeated pressures beyond 50ksi in a close fitting, strongly made action which does not demand that the case serve duty for which no cartridge is designed.

Well, if that's so, how does any cartridge fail? Well, when the smoke clears, it's got little to do with the cartridge, and everything to do with the gun.

Within practical limits, relatively poor execution in design or construction can result in few or more guns that are predisposed to cartridge failure.

I imagine everyone here has at some point seen "Glock Bulge". I don't know whether the current generation of Glocks are prone to this, but for years it was possible to identify a spent casing fired in a Glock by the pronounced bulge at the base of the case. This was caused by the barrel having an excessively long ramp that did not support the rear of the case adequately. Case manufacturers silently increased the thickness of cases because the popularity of the Glock was the cause of a significant increase in ammo sales.

Something that cartridge makers can do nothing about is excessive gap at the breech. At ignition, primers are typically driven out of the case head and against the breech. The main charge then drives the case back over the primer. When things are bad enough, cratering will be noted on the fired primers, which will also be quite flat and impressed with the breech surface texture (A sure way to match an expended case with a gun). When things get even worse, gases may escape around the primer, producing a characteristic dark halo around the primer - A sign of impending disaster.

I mentioned that the cartridge case is forced back over the primer by the main charge. Here is where all sorts of things can go wrong. Depending on how large the gap is between the base of the cartridge and the breechface, and how much additional unsupported case length there is, this is where a case blowout can occur, releasing the complete charge through the action of the gun. Again, this has little to do with the relative strength of fresh factory brass.

A blowout can consist of a pierced primer, the failure of an unconstrained bulge, or the separation of the head of a case at the web transition. All of these require some sort of shortfall in the gun itself. Excessive distance from the base of the cartridge to the breechface, whether due to excessive headspace in a revolver or pistol, or cylinder endshake in a revolver, or poor barrel throat support in a pistol, are the likely causes.

Fortunately all of these conditions can be read from a fired case. Reading them is another topic for when I feel particularly motivated. Or you can read Phil Sharpe's Complete Guide To Handloading for yourself.

· Uno Ab Alto
421 Posts
I enjoyed the discourse, pokute.

However, what about the other primary purposes of the cartridge, i.e., portability, quick reloading vs. one-shot power & ball, maintaining dry powder, etc... ?
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