Has anyone ever have brass not strech in a simi-auto AR-15?

[Blue68f100]

Back on topic.

I cleaned (wet tumbled) up a batch (200+) of my 224V brass yesterday. Looking at my log this brass now has 6 firings on it.

This morning before I did anything to the brass I put a few through my chamber gauge. I'm getting less than 0.001" stretch, ~ 0.0005"-0.0007" average. My depth gauge only reads to 0.001" so any thing less is just a guesstimate. I did force these in by finger pressure to make sure they were all the in before taking the measurement. All it took was a couple of taps on the bench for the brass to fall out on it's own. Considering these were all fired in a gas operated simi-auto (rifle length gas system), and ejected while still under pressure, there is very little stretch. I use this same chamber gauge when I setup up my sizing die.

The chamber gauge is set so the top of the gauge is the Go Gauge length, <-0.0005 +0.0000. The NO-Go is above my gauge. So any ammo that fits this gauge will fit any chamber. I size all my chambers this way and this is the first one that the brass basically does not stretch on.

I'm getting to the point now where some of the primer pockets are getting loose. I'm retiring those I run across them.

My guess is that by pure luck, I have the buffer,spring and gas system as near perfect as one can wish for.

The whole reason I started this thread, was that I was wanting to true the neck face up from the factory. Since all the brass came in at trim length, I figured after several firings I would be able to trim them. Not the case, I really don't want them to any shorter.

If your don't believe that a AR bolt does not have enough energy to size the brass. I would suggest you size one measure it, let the bolt slam home, eject, and remeasure again. It is limited to the amount of space that was setup between the barrel and the barrel extension nut. This is a function on how generous your chamber is or is not. On a loose chamber I dought you will see a change. On a tight chamber and a round not sized quite right it can.

 

[fguffey]

I’ve always considered the FP crush fitting thing to be a red herring. Mechanically, it doesn’t suit that a firing pin would deform a large area (shoulder surface) instead of a small area (FP dimple in the primer cup).

You must have thought of that before anyone else did. We have members on many forums including this one that insist the firing pin drives the case to the shoulder of the chamber with enough force to shorten the case from the shoulder of the case to the case head .005". I have disagreed; nothing personal but when trying to determine what happens to the shoulder when sizing and firing I have too many rifles that prove it is impossible to move the shoulder forward when firing. The same for the shoulder when sizing. I find it impossible to move the shoulder back with a die that has case body support.

Deforming a case with impact; I have driven cases into chambers with a long drift and light hammer. If the case is deformed when driven into the chamber and I want to know by how much I should have measured the length of the case from the shoulder of the case to the case head AND the diameter of the case at the case body/shoulder juncture. AND I have used arbor presses and no I do not forget the case has no chance of having enough resistance to require an extension of the handle.

F. Guffey