The 2 1/2" refers to the hull length after firing, so an unfired 3" shell might fit. 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.

There are disassembly and reassembly instructions in the FAQ "Maintenance and Restoration" section. The hammers need to be cocked, and if not there are instructions on how to do so with the barrels removed.
You do NOT want to remove the stock from the action as reassembly is a challenge
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