Why did my brass get shorter after once fired and FL resizing?

7.62x39 Lapua brass new 1.515in long. Once fired, full length resized, all resized brass measures the same from head to shoulder using the Stoney Point Head and Shoulder Gauge.
Most of this brass has an overall length of 1.515in. or a bit more but, several have done got smaller. They measure anyware from 1.508 to 1.511.
Where did my brass go? The distance from head to shoulder is consistent, but, the overall length has shrunk.

Measure one thats been fired and not resized and you`ll see more shortening.
I think what you are seeing is the case shorten when being blown out to fill the chamber. The die sizes the case back down but, not all the way to the same diameter as it was when new. The displaced brass is still there just in the wider body and not lenght. This is a good example of why one should resize their cases BEFORE trimming. The case lenght will vary with resizing.

I agree with Ol' Joe I think your chamber is a hair big around the length is probably ok. What is the gun your using these in and what is the make of your dies?

quote:

Originally posted by Jay Johnson:
I agree with Ol' Joe I think your chamber is a hair big around the length is probably ok. What is the gun your using these in and what is the make of your dies?

Ruger Mini-30, and my Dies are RCBS FL dies.

Thanks

I think the guys who told you your chamber is slightly oversize (diameter) are probably right, considering the rather short overall length of that particular cartridge.

With some longer cartridges, say of about '06 length, just an oversize NECK will also sometimes measurably shorten the case when fired in that chamber.