Please do not get too disheartened Lloyd.
Whilst the British gun trade regards 20 thou minimum wall thickness as “minimum recommended wall thickness“ this is more with a view to having sufficient metal to deal with raising dents and knocking down bulges than as a concern about the possibility of bursting.
Quite a few very light guns, particularly 2” guns, seem to have left the factory with MWT in the high teens.
Remember that a bore enlargement of 7 thou, assuming concentricity, is only a reduction of 3.5 at each wall, and that MWT being close behind the muzzles will be at a point where the pressure curve for a shotgun cartridge has fallen to a small fraction of peak pressure.
As you have a minimum of 20 thou you should be fine with any 2 1/2” cartridges loaded to CIP pressures.
But remember the rule of 96, and out of respect to your shoulder and the stock don’t use more than 1 ounce in a 6 pound gun.
Season’s Greetings
Parabola
