Standard 12 bore “paradox” cartridge length was 2 1/2
Long range and magnum 2 3/4 inch

Certainly 12bore rifles and heavy ball guns could have been chambered at 3 1/2 but wouldn’t have been considered a paradox style weapon

But have to have more info on the gun

That’s why he’s having trouble finding info on 3.5 inch paradox load there were none.