Subsonic 308 reduced case length after range

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Mariot320

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Any one loading subsonic 308?

I've loaded my first 5 rounds of subsonic with new lapua brass, FL sized,and CASE LENGTH 2.005". I went to the range and to test for stability and key hole before I bought a chrono... I checked Case length when I got home,and they measured anywhere from 1.999" -2.001". They reduced in sized after shooting them.

Can some one explain why this is? It seems I can't you reuse these once fired cases since they shrunk below minimum CL.. Did I do something wrong? Any input and advise is appreciated.
 
Your chamber is normally bigger then your loaded ammo.

When you fire it, pressure expands the brass to fit the chamber.
So it gets bigger around and shorter.

Once you re-size again, it will get smaller around, and longer again.

You can't really measure case length on fired brass and tell anything from it before you re-size it.

rc
 
Welcome to THR. First off, I wouldn't have done anything to new Lapua brass other than remove any dents in the mouths. How confident are you in the measurements? Did you need to trim the cases before use? I have a hard time with the idea that a case can shrink after firing even with subsonic pressures.
 
That makes sense ! Just wanted to clear that up before I moved forward and waste brass if there was a problem... I'll FL size them , and I guess neck sizing is out the question..
 
1858 said:
Welcome to THR. First off, I wouldn't have done anything to new Lapua brass other than remove any dents in the mouths. How confident are you in the measurements? Did you need to trim the cases before use? I have a hard time with the idea that a case can shrink after firing even with subsonic pressures.
Click to expand...
I'm having a hard time with it too ...I FL sized and pretty much hand picked the ones that were 2.005" in case length. I'm shooting through my rem 700 aac-sd btw.
 
No, you can still neck-size them.

You just should't have trimmed them before firing and neck or FL sizing them to see if they needed trimming after fire-forming to Your chamber, and being re-sized in Your die.

Even if they come up short after neck-sizing, it won't hurt anything except your pride! :D

rc
 
rcmodel said:
When you fire it, pressure expands the brass to fit the chamber.
So it gets bigger around and shorter.
Click to expand...

I just measured a bunch of new and fired cases (same lot) shot in my AR308. The new cases are 2.006" and the fired cases range from 2.014" to 2.019" in length. This is after only one firing and no resizing. These are supersonic loads that were shot with a suppressor attached to the rifle.
 
I have measured brand new brass and noticed it was shorter than the recommended trim length. I also have noticed it gets longer and narrower after full length sizing it.
 
Mariot320,
I've been thinking about this and have a theory as to why your cases "could" reduce in length. rc made the point that the case expands and gets shorter but my .308 Win cases expanded and got longer by approximately 0.012" but yours reduced in length by approximately 0.005". The difference is that my cases were exposed to an internal pressure around 62,000 psi (supersonic) whereas yours to a pressure considerably less and possibly close to 36,000 psi (subsonic). There's a lot more radial surface area inside a case than axial so a case expands radially first until it's limited by the chamber wall. During this stage the case is getting shorter. If there's sufficient pressure and the case is pushed up against the chamber wall the only place for the brass to flow is forward or axially. This is how shoulders move forward. New .375 H&H Mag cases shot this past weekend in my Talkeetna have a shoulder that is almost 0.010" further forward than the original new case. The overall length of the cases increased too. So basically, my theory is that you had enough pressure to expand the case against the chamber wall which shortened the cases, but not enough to cause the case to flow forward. Measure your shoulder location of new and fired cases to see if the shoulder moved at all. This is just a theory so take it for what it is.
 
When the primer goes off, it pushes the case forward. Typically with low recoil loads, the pressure is not high enough to stretch the case, just high enough to drive the case back toward the boltface and drive the primer back into the pocket. If you took one of your cases and loaded it with just a primer with no powder and no bullet, then shot it, you'd see the primer sticking out of the pocket slightly. That's because your headspace is a little longer than the headspace of the case.
 
Post 9 is what happens. Its a pressure thing. Some autos, M1a (?) May open the action to soon, allowing the brass to get longer then the chamber when measured head to datum. The firing pin strike from a Savage Axis in 223 will set the shoulder back .006"
 
243winxb said:
Some autos, M1a (?) May open the action to soon, allowing the brass to get longer then the chamber when measured head to datum.
Click to expand...

Good point. It prompted me to measure the shoulders on the fired cases mentioned above using a Redding instant indicator case comparator. The new cases are around 0.005" less than the SAAMI minimum chamber and fired cases are around 0.005" to 0.007" longer than the SAAMI minimum chamber. I don't know what the headspace is in my POF chamber but I doubt it's SAAMI minimum. If it is at the minimum dimension then clearly the cases are being stretched during extraction.
 
Mariot320 said:
Any one loading subsonic 308?

I've loaded my first 5 rounds of subsonic with new lapua brass, FL sized,and CASE LENGTH 2.005". I went to the range and to test for stability and key hole before I bought a chrono... I checked Case length when I got home,and they measured anywhere from 1.999" -2.001". They reduced in sized after shooting them.

Can some one explain why this is? It seems I can't you reuse these once fired cases since they shrunk below minimum CL.. Did I do something wrong? Any input and advise is appreciated.
Click to expand...

Could you please provide the load data as I am doing the same thing with the same gun. Thank you.
 
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