I recently went to the range and used some older ammo. It's Winchester white box practice ammo in .357SIG. I'm thinking I bought it in 2004 or maybe earlier. I am almost certain that it's factory ammo but there is a chance that I mixed in a reload with the new stuff. It didn't look reloaded as I used different bullets in my reloads.

Everything fired without issue but when going through the brass to separate the calibers, I came across this one:


None of the others cases have an issue, only this one.

I showed a friend and he brought up "cold welding". I looked it up and it seems that the ammo would have to be much older than 20 years for this to occur. What else could have caused the case to split like that?

Over the decades and many(!) thousands of rounds I’ve fired, I’ve experienced perhaps four or five split cases like that. They were not handloads and weren’t particularly old. The earliest one I recall was from a box of factory ammunition that was taken from the store to the range, and according to one long discussion I found, it’s not supposed to be an issue with factory ammunition, including military stuff that can be many decades old, because of how it’s processed.

If the split case was due to excessive pressure, I would think that you’d have noticed something when you fired it. Also, the one account I read about said that the neck splits and separations occurred with numerous rounds from a handloaded batch, not just a single example.

I suppose it was possibly due to the phenomenon, but I believe it was much more likely due to a bad case for whatever reason it happens: metallurgy, annealing, gremlins, .... Or it could have just been caused by a high pressure load without being the fault of the case itself. The 357 SIG cartridge can be loaded to the higher end of SAAMI specifications, and perhaps the bottlenecked case is somewhat more susceptible problems than others (although I’ve never experienced any problems myself in thousands of rounds).

As a final comment about splits like that, unless the chamber pressure is high enough to blow up the gun, the usual concern is that the gas escaping through the split can erode the inside of the chamber, causing roughness and ultimately extraction problems.

Thanks. It’s only the one case out of 20 or so. I’ll just mark it down as a fluke.

A simple scratch on a case will cause that if it's deep enough. As for how a case could acquire such a scratch a metal fine lodged in to the die on an automated reloading line can cause this and if embedded in the die cause many thousands of this defect.

Cold welding is galling, when two similar metals friction weld together, primarily an issue in early stainless steel pistols.

I think what your friend meant was cold-working, which is when you do something like bend sheet metal back and forth until it breaks along the fold.

You often see cracks in case necks after repeated resizing cold works the brass, which annealing helps reset the crystalline structure to be more ductile. In new brass I would guess defect, chamber issue, cycling mis-timing, overpressure or previous case damage.

When I first got my .17 HMR I bought a bunch of ammo and found many cases cracked straight from the factory.
So you might just have a case that was defective from the start, normal pressure was enough to cause the crack.

The only cases I've seen with that sort of split were all chrome plated, and most were .357 Magnum. Chrome is hard and slick, but it can embrittle the metal it is plated to.