i always thought 3 tons per square box (sq. inch) under imperial measurement was analogous to 900kg/sq.cm proof under the si for guns with 65mm chambers and 3&1/4t/"sq or 900kg/sq.cm for those having 70mm chambers.
prior to '89 76mm chambered guns were proofed to 3&1/2t/inch sq. or 1200kg/sq.cm.
theoretically one can shoot 65mm lead cartridges up to 1&3/16oz in three toner (it's probably old so i would stick to 1oz cip rated shells), but i would not put some american gizmo in english gun and shoot 28bore cartridges through it because that isn't cartridge for which this gun was intended. i'm sure there is a warning on box flap of those 28ga cartridges so their ass is covered and you're doing this at YOUR OWN RISK.
