Well, I might have a problem.
So, let's kick it around.
Nominally, the shotgun is a 10g.
I finally found some 10g shells. Remington RP, the green plastic ones, high brass.
Purchased the low-base wads, 10g.
First, I dropped a plastic shotcup/wad down each barrel, base-first. The base didn't touch the walls until almost to the muzzle.
Second, I trimmed a shell to 3", expected it to be a quarter inch too long. The forcing cone starts at 2.70" from rear of chamber, tapers to barrel dimension at 2.75" - but the shell slid all 3" into the barrel and extracts like butter. I can tip the barrel up and let the hull slide out.
This means both shells inserted past the forcing cone, which means the barrels (just past the forcing cones) is the dimension of the outside of the 10g shells.
Third, I sent an untrimmed 3.25" shell into the chamber, and it went in almost all the way before it touched the barrel sidewalls.
I think, that I have a 9 gauge? Chamber measures .853 where the brass base inserts.
Q1. Is it safe to "fire-form" the RP shells to these chambers?
Q2. Should I trim the cases back from 3.00" to 2.700"