Builder;
Stepped chambers did indeed exist, & for paper shells also. I am not personally knowledgable as to whether or not Parker used then. Greener discussed them & stated shell lengths were not controlled close enough to provide the desired advantages & they left a stress point & also were hard to clean right at the point of the stress riser. They are most apt to be encountered on extremely old guns & any gun of this age should be given a through examination prior to firing with anything, even black. This is certainly not meant to imply they should never be fired, only to proceed with utmost caution. I would certinly think one of these guns chambered with a sharp step should never be used with shells having a longer than chamber fired length, even though they were shorter in their loaded length.

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