Negative Headspace

[mike06]

I have a Ruger Precision Gen 2 and two Rem 700's 5r all in .308. I take a new or a resize piece of brass with zero reading on my RCBS head space gauge and after firing the cases will come out with a -.002 negative head space. None are considered as a hot load. Any ideas what could be causing this?

 

[AJC]

Your chamber may be large in diameter and the brass goes out.

 

[bozo699]

I have a Ruger Precision Gen 2 and two Rem 700's 5r all in .308. I take a new or a resize piece of brass with zero reading on my RCBS head space gauge and after firing the cases will come out with a -.002 negative head space. None are considered as a hot load. Any ideas what could be causing this?

Mike,
I have never used the rcbs tool but it sounds to me like your saying it’s stretching.002 which isn’t bad at all in my thinking.
Wayne

 

[jelenko]

He's saying it's coming out minus 2 on the RCBS headspace gauge. I.e., since the brass started at 0 and came out -2, it appears to be shrinking.

First question is what effort is needed to chamber the round?

In any case, even if the chamber is close to 0, won't it come out a bit smaller due to spring back?

 

[AJC]

He's saying it's coming out minus 2 on the RCBS headspace gauge. I.e., since the brass started at 0 and came out -2, it appears to be shrinking.

First question is what effort is needed to chamber the round?

In any case, even if the chamber is close to 0, won't it come out a bit smaller due to spring back?

If the case is smaller than the chamber it will expand to the chamber walls. If it was a low power load or the headspace is at sami spec it may not grow.
There is no negative headspace per sei, but a crush fit. The case shrank in length after being fired.

 

[Metal God]

You mentioned 3 different rifles .

1) Does this happen with one or all of them ?

2) Is this only one load that does it or all your loads ?

 

[mike06]

You mentioned 3 different rifles .

1) Does this happen with one or all of them ?

2) Is this only one load that does it or all your loads ?

All loads all 3 rifles happens with all brands of brass. Lapua, Winchester, Rem and Peterson

 

[Fast14riot]

Could be a change in shoulder angle. Could use a piece of .40s&w brass to check, too, the datum on a 308win case is .400". If the shoulder angle changes just a little, like when sizing brass with an expander button but not bumping shoulders, you'll get longer cases after sizing. If your filling out the shoulder of the case during firing but not enough to cause case growth, you could see the apparent shrinkage.

 

[Texas10]

Ever notice how your shirts seem to get shorter as your waist gets bigger? LOL Same thing going on in the chamber. Case initially expands to fit chamber, it has to shorten somewhere, so it shortens in length. After a couple more cycles It'll be fully fireformed and fit snugly. That's when I will start with FL sizing, but not before.

 

[Metal God]

I like the waist band idea but I don’t think so here . Every rifle and every load ???????

This seems very odd to me . Maybe one with the others but not all . I can see one chamber being fat and every load thats fired will bloat the sides and suck the shoulder back a bit .

Or

A light load not having enough pressure to full fireform in any rifle .

But every load regardless of charge in every rifle results in the case from head to datum being shorter ??????

This seems odd to me . There’s got to be a factory load or reload out there that will blow the case out close to fully fire formed.

This Sounds like maybe the primers aren’t fully seated so he’s getting that extra couple of thousands when measuring before firing. I’m also not opposed to the shoulder angle changing thing giving him slightly different readings . The dye or chamber shoulder is cut slightly off ????

I’m gonna have to put some more thought to this because I can usually figure this stuff out rather quickly but this one’s got me stumped a bit . :-(

Edit

Oh what about Akeley improved Die or chamber ? I forget which one can be what but can’t you shoot one in the other ? That would change the shoulder angle ?

Yeah I know , it's later and I’m reaching now haha

 

[Marksman63]

I use a RCBS precision micrometer for headspace reading. The RCBS’s zero is just a reference; it does not mean is the correct “zero” headspace. Try to resize your brass with -00.2” headspace referred to your current reading. Shoot them and measure them again. That reading is your “zero” headspace

 

[Richard Coody]

Well if you really want to solve this. If it is truely a problem for you then the solution is custom sizing dies.

If this still happens after 4 or 5 firings you are obviously oversizing the body of your brass. Custom dies made from your fully fireformed and unsized brass will not do this.

 

[Richard Coody]

I use a RCBS precision micrometer for headspace reading. The RCBS’s zero is just a reference; it does not mean is the correct “zero” headspace. Try to resize your brass with -00.2” headspace referred to your current reading. Shoot them and measure them again. That reading is your “zero” headspace

Most reliable way to ensure your brass is fully fireformed is when you can take a piece of fired and unsized brass and chamber it and the bolt closes with resistance. If it chambers easily then your brass isn't fireformed and you are not ready to set up a shoulder bump on your die.

 

[dolek]

Most reliable way to ensure your brass is fully fireformed is when you can take a piece of fired and unsized brass and chamber it and the bolt closes with resistance. If it chambers easily then your brass isn't fireformed and you are not ready to set up a shoulder bump on your die.

+ 1 Take this approach and you will learn all you need to know .

 

[BDman]

A friend of mine experienced the shrinking headspace problem. It stopped after I convinced him that he did not need small base dies.

 

[Ringostar]

I use a RCBS precision micrometer for headspace reading. The RCBS’s zero is just a reference; it does not mean is the correct “zero” headspace. Try to resize your brass with -00.2” headspace referred to your current reading. Shoot them and measure them again. That reading is your “zero” headspace

This is what I was reading here also. It sounds like a measuring issue.

 

[Dusty Stevens]

id say try a different measuring tool to verify. Those type measuring devices need to be zeroed with a case thats been fired at least 3 times and set as your zero position

 

[DaveTooley]

I quote

"I take a new or a resize piece of brass"

The brass stretches then springs back. A little work hardening will stop that.

 

[6MMsteve]

I set my headspace tight on my salvage rifles a little pressure on bolt closing bout 2/3 down. I get the shrinkage for a couple firings then it stops. I was puzzled at the time also. I use the horny shoulder gauge with Mitutoyo IP67 cals which are dead on in repeatability smooth as glass

 

[mike06]

I think I will just neck size until the bolt gets tight then go from there and just have a test case for each rifle. I just use the RCBS tool to get a starting point idea of what is going on once I get the case where I think it should be the I set up my Redding tool with a Digital gauge to get a more precise measurement. Thanks for all the input guys.