"OLD" SAAMI specs for the 12 gauge 2 3/4" shell sho a max angle from the horizontal axis of 5 for the chamber cone. With standard chamber & bore sizes, this will give a cone length of about 0.400", or just over 3/8".

They also showed a minimum chamber length of 2.616" or a bit under 5/8".

You were "Very" wise to trim those shells for use in the early (I assume with that chambering it was early) Parker with a stepped chamber. It likely was made for the short 2 58" paper shells. Brass shells have thin walls & require oversized wads. A stepped chamber would be a decided disadvantage with them. NO shell should be fired which allows the end of the shell to lap into the bore itself upon the opening of the crimp.

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