Agree with Joe.
As you go down the chamber you should be getting a gradual thinning of the wall, then as the cone is contacted there would normlly be a quick reversal as the wall gets thicker upon entering the cone. A careful study of this will also reveal why lengthening a chamber results in a thinner wall, but the cone can be lengthened considerably without a resultant thinning.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra