I think youre going to find that the chamber, if unaltered, is 2 5/8, not 2 1/2. And Parker, along with LC Smith, and Im sure others continued to cut this length chamber long after 2 3/4 hulls became standard. They wanted the hull to open part way inside the forcing cone, believing that a hull mouth inside the cone provided a better gas seal. Makes sense. I do know that Parkers hang tag that came with a new gun said, chamber 2 5/8, use 2 3/4 ammunition. Most of my guns have short chambers and I never cut the hulls, theyre all 2 3/4 and Ive never had a problem.

I see from the photo of your reloading room youre a rifle reloader and that does require a high degree of precision in case length, etc. but shotguns are totally different animals. Such precision is not necessary. But the combination of primer, powder, and shot charge are critical. Not hull length.

Enjoy your new gun, sounds nice!

When an old man dies a library burns to the ground. (Old African proverb)