When Does?

A brass cartridge case grow shorter after its been fired in a center-fire rifle.

Just wondering what results for those who check their brass prior to de-capping and beginning case prep.

Cases can get shorter after firing, called fire forming(case body expands). As the case expands to match chamber, your neck pulls back. Typically after 3 firings it will stop happening.

I'll usually try to leave newer brass long bt a few thou for first 3 firings.

I have found, the more reloadings a piece of brass gets, the less I trim off it. I usually don't reload a piece more than 3 times. I occasionally come across 'short' brass and just toss them in the recycle bin.

Agree, just like in a modified or improved chamber. But, do you think its actually the neck contracting or the body of the case expanding? It is generally agreed upon that the flow of brass is forward, the same direction additional chamber clearance is usually oriented. What op-positional forces, if any, can be created to sort of create a hold point.
The only thing I can personally think of is a powder with more quickness.

I believe that it's the shoulder matching the chamber and sucking the neck backwards at the same time.

Maybe Ed shell or park ranger will chime in, those guys have more experience that I do.

Thats exactly what Im thinking. The case is head-spaced properly, but the shoulder reacts that way because the case body has already gripped the chamber walls at the rear and essentially its fire-forming to a different shoulder angle.
Thanks.

In my experience, if a case is fired that is not properly formed to the chamber (the case is smaller/shorter), it can get longer or shorter, depending on several factors.

If the case indexes back against the boltface when fired, it will blow the shoulders out forward to the chamber, using up brass at the shoulder, and shortening the case length. This is how you should fireform a case to determine correct dimensionality and shoulder headspace. This can be accomplished by loading a case with the bullet well into the lands with fairly tight neck tension (may have somewhat stiff bolt closure, and don't remove before firing), or by using a very light film of a lubricant on the case. Or both.

If the case indexes on the shoulder when fired, leaving space between the boltface and case head, the case may expand backwards, using (stretching) case sidewall brass to fireform to the chamber length. This is usually not so good, as eventual case separation may occur with repeated loading/shooting. A visible line or striated section can sometimes be observed in the stretched length of the case.

I recently did this simple test at the range with a shooter who was experiencing case separations with his .308 handloads. He fired one of his reloads with no lube. The case length got longer. Then he fired another with a light coating of Kroil. The case got shorter. The solution is to reset the sizing die to not knock the shoulder back as far as he was doing previously.

Are we talking about cases reloaded for bolt guns or semi-auto guns. There's a big difference. Almost all my reloading is done for semis.

I have done limited testing with my AR-15's, 2 are .223 Wylde and one 224Valkyrie. Same seems to hold. I resize for the AR's a bit "looser" than for bolt guns.

Where does the majority of 'stretch' come from? The neck or shoulder or down around the case head? Does it occur throughout the length of the case?

outrider58 said:

Where does the majority of 'stretch' come from? The neck or shoulder or down around the case head? Does it occur throughout the length of the case?

Click to expand...

I have one rifle that will not close on a field gauge. With light loads it will sometimes very slightly raise the primer above the head of the case. With a 2 grain increase to the charge the problem does not exist. I suspect the body of the case has an increased grip on the chamber walls and then in turn reduced bolt thrust.
The case does not grow shorter or have a reduced length even when fired from an oil damp or "wet" chamber.

I use another a rifle that head spaces on the belt and not the shoulder. The cases always grow longer until they are just neck sized and then do not often require trimmed. This rifle has a SAAMI chamber, dies-brass.

Other (multiple) rifles No SAAMI chambers but SAAMI brass, grow longer when fired and with a shoulder that moves significantly forward. These are rimmed cartridges and when neck sized have been hand loaded multiple times. On these cases every once in a while a case will separate which is indicative of brass moving forward. Necks will also split usually around the 10th or 12th time they are loaded depending on which batch they are in or who made them. WIn cases are the worst offenders for variance in rim thickness between different lots with this particular ammunition.

I have another rifle (1) with brass that grows shorter until sufficiently flowed out from what I suspect is the base of the case. Its a rimless case. When re-sized the die is adjusted to size the neck and just touch the shoulder so the stripped bolt just has perceptible residence when being closed. It is not a SAAMI chamber. The brass when new has a longer shoulder than the fired brass but the same angle. I'm going to make a cast of the chamber to see exactly what the case taper is vs how the chamber taper measures out at which is what direction I'm considering right now.
Because brass is available its not worth trying to extend the useful life of it past probably 4-6 times. I have no idea how the brass is flowing /moving or what from direction until I can measure it correctly, its like a 42000 psi cartridge with a minimum published load of 4064