The U.S. makers decline to provide pressure data, and with the exception of Tom Armbrust, those doing ballistic testing won't test factory loads.
Most of the Italian makers do.
B&P Comp One 28 gm is only 6530 psi
IMHO it is likely your 12 g chamber is 2 5/8"
There is an easy way to measure chamber length using a 3" x 5" index card, rolled lengthwise, and slid into the chamber. It will partially unroll and expand to the chamber diameter.
Gently advance the card until it stops at the end of the chamber (where the forcing cone constriction starts), use a pencil to mark the card at the breech end of the barrel, and repeat the steps to see if it always comes to same spot.
Then use a ruler to measure the length marked on the card.
The chamber must be clean, and it works best with a new 3” X 5” card.
Let us know what you find.
If 2 5/8" (and I recognize we're quibbling over 1/8") pick a 1 oz at 1200 fps, shoot and be happy.
It's more difficult with 16g and 20g loads.
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