Negative Headspace

[paperpuncher]

I think this has to do with how the gauge is matching the shoulder. The shoulder is likely changing a bit after firing and then you get a different reference point

 

[6.5grendel]

I noticed this same same thing when I started reloading. The fired case would come out with a shorter headspace (distance from base to shoulder datum) than the sized or new case. The issue was exactly what most people have already said. The diameter of the chamber is a little larger than a sized case and when the case expands in the chamber it expands in diameter before it grows in length. When the case expands in diameter it also shortens the length of the case slightly. It can take multiple firings for the case to expand fully to the headspace dimensions of the chamber especially if using lower pressure loads. There were two ways that I found to make them expand. I worked up a ladder to find max pressure in that rifle, then loaded the cases with a load a little under max pressure. That caused the shoulders to grow forward. The other way was to neck size the fired cases without full length sizing them. Then the case was already expanded to the width of the chamber and only had to grow length wise. That was just my experience. Yours might be a different reason, but I would bet it's the same.

 

[shoot4fun]

So you have three different rifles; two competing brands. But you're telling us you have the same readings, before and after, on brass fired in any of those three rifles? I don't see that is a million eons.
What I do see is the common element in all the measurements and that is the tool you're using to measure them.

 

[DaveTooley]

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.

If possible take one piece of brass and reload it several times. Just neck sizing. As soon as it starts to get the least bit tight switch over to a FL die. Get the shoulder bump you want then throw that piece of brass away. That piece has work hardened to the size of the chamber and will not respond the same as fresh brass.
If you wait until all your brass is tight it will be nothing but problems from then on.

 

[msinc]

All this measuring and suggestions of size and measure again, but not one word about a headspace "Go" gauge. Size and check against what standard???
I get that the RCBS case mic should be correct, but what if it's not. I chamber a lot of barrels and I always set the headspace to just feel the Go gauge when closing a stripped bolt. Without exception, a case sized to zero {minimum headspace length} which is the same as the new chamber, loaded and fired will always be .001"-.002" longer than the chambers minimum length.
I have yet to see one "shrink". Not saying it cannot happen, but I am saying I have never seen it...I will also say I don't make chamber dimensions anything other than SAAMI, so no oversized ones.
I think if you really want to get to the bottom of this you might want to get yourself a "GO" gauge and start with the case mic. I have seen several that were not the same zero as a gauge. I don't believe RCBS let them get out that way, I think people horse down on them...we are only talking about .001". Gauges and precision measuring devices need to be handled with a light touch. Many folks don't seem to have this.
Next I would start checking the chambers themselves. With a Go gauge you have a confirmed starting point, with fired brass you have a close guess. Can you still load and shoot the guns safely without it??? Sure, but you still wont "Know" what's really going on.
I mean, size the -.002" cases to .002" under that and I bet they come out back to -.002", but you still wont know if its your chamber or your case mic.

 

[Metal God]

The gauge doesn’t need to be correct because it’s being used as a comparator . My Hornady headspace gauge/comparator measures my GO gauge .010 shorter then it is . That’s not a big deal I just measure the go gauge zero the calipers then measure the brass . This will tell me how short or longer the brass is compared to my gauge . Same thing can be done with a unfired and fired cases and comparing one to the other . The goal is not to get an absolute measurement But rather comparing one to the other

 

[jelenko]

All this measuring and suggestions of size and measure again, but not one word about a headspace "Go" gauge. Size and check against what standard???
I get that the RCBS case mic should be correct, but what if it's not. I chamber a lot of barrels and I always set the headspace to just feel the Go gauge when closing a stripped bolt. Without exception, a case sized to zero {minimum headspace length} which is the same as the new chamber, loaded and fired will always be .001"-.002" longer than the chambers minimum length.
I have yet to see one "shrink". Not saying it cannot happen, but I am saying I have never seen it...I will also say I don't make chamber dimensions anything other than SAAMI, so no oversized ones.
I think if you really want to get to the bottom of this you might want to get yourself a "GO" gauge and start with the case mic. I have seen several that were not the same zero as a gauge. I don't believe RCBS let them get out that way, I think people horse down on them...we are only talking about .001". Gauges and precision measuring devices need to be handled with a light touch. Many folks don't seem to have this.
Next I would start checking the chambers themselves. With a Go gauge you have a confirmed starting point, with fired brass you have a close guess. Can you still load and shoot the guns safely without it??? Sure, but you still wont "Know" what's really going on.
I mean, size the -.002" cases to .002" under that and I bet they come out back to -.002", but you still wont know if its your chamber or your case mic.

I bought a go gauge for exactly this point.

 

[mike06]

All this measuring and suggestions of size and measure again, but not one word about a headspace "Go" gauge. Size and check against what standard???
I get that the RCBS case mic should be correct, but what if it's not. I chamber a lot of barrels and I always set the headspace to just feel the Go gauge when closing a stripped bolt. Without exception, a case sized to zero {minimum headspace length} which is the same as the new chamber, loaded and fired will always be .001"-.002" longer than the chambers minimum length.
I have yet to see one "shrink". Not saying it cannot happen, but I am saying I have never seen it...I will also say I don't make chamber dimensions anything other than SAAMI, so no oversized ones.
I think if you really want to get to the bottom of this you might want to get yourself a "GO" gauge and start with the case mic. I have seen several that were not the same zero as a gauge. I don't believe RCBS let them get out that way, I think people horse down on them...we are only talking about .001". Gauges and precision measuring devices need to be handled with a light touch. Many folks don't seem to have this.
Next I would start checking the chambers themselves. With a Go gauge you have a confirmed starting point, with fired brass you have a close guess. Can you still load and shoot the guns safely without it??? Sure, but you still wont "Know" what's really going on.
I mean, size the -.002" cases to .002" under that and I bet they come out back to -.002", but you still wont know if its your chamber or your case mic.

I have a set up gauge that came with my Redding Comparator Instance Indicator and the RCBS tool comes up with zero head space using that setup tool.