I would not do anything to alter the barrels, ever, especially barrels belonging to an English gun. It’s hard to find English guns that haven’t been messed with. Brits know how to take care of their guns and refinishing didn’t effect the value like it does with American guns, but any alteration of the barrels will reduce the value of a British gun.
Lengthening chambers will let you shoot 2-3/4” ammo, but at some point low pressure 2-1/2” shells will be available, but your barrels will still have been modified.
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