Your gun was shipped in 1945; If your gun is a 12-bore, chamber length will be 2 3/4" from the factory; If you plan to shoot ducks with a vintage double gun, shells loaded with Bismuth shot are recommended (it is illegal to waterfowl hunt with shot shells loaded with lead shot, and steel and tungsten matrix loads are not recommended for vintage doubles); As to max load, assuming a solid stock, tight screws, good mechanical condition, etc; your gun should handle 12-bore loads of 3 1/4 dram equiv, and 1 1/4 oz shot OK; but anything heavier than 3 1/8 dram equiv. and 1 1/8 oz will be rough on your stock (most Smith gun users will handload shells to low pressues less that 8,000 PSI).