Spent cases are an excellent indicator of chamber conditions. If you start with a new case and fire your load, the case will fire form to the chamber.
This is NOT used as a pressure indicator but rather a snap shot of the chamber itself. Keep in mind, the case head is solid except for the primer pocket and flash hole. It will not expand much even in an oversized chamber. Expanded head areas will show up when measuring brassl thickness at the point where the web meets the case wall.
The spent case will reveal all sorts of information. If you clean your case really well before firing, the brass will show any irregularities such as pits, rust, scratches, and machine marks. Further, if you use a RCBS CaseMaster Gauging Tool, you can measure the thickness of the brass from the web to the mouth. Case run out can be measured and will indicate an essentric chamber. The outside diameter of the brass can be measured in any location to indicate expansion. The most critical area is the point where the case wall meets the head (edge of the web). This can also be measured with the CaseMaster. Of course you need to compare the case with an unfired case to see where the differences are.
All cases will expand so don't get excited if you see a couple thousandths difference between a new case and a fired case. What you are primarily looking for is thin areas in the brass where the case has stretched. This is a very positive indicator of an expanded chamber.
See:
http://www.rcbs.com/default.asp?menu=1&s1=4&s2=3&s3=29I have one of these tools and use it to troubleshoot rifles that don't group well. Often I find an essentric cut chamber where they don't line up with the bore perfect. I also use it to inspect cases after they have been fired a few times. The included probe will measure for thickness at the case wall/web where head seperation is likely. I also check runout on my reloads. Once in a while you will find a case that wasn't made right and will have runout no matter how good your dies and reloading techniques are.
