6720psi is simply a multiplication of 2240 lbs (British Long Ton) by 3 tons psi. This does not give a proper conversion to modern day psi's. 3 ton crusher pressure when given as psi by modern piezoelectric gages will read very near to 9,000 psi. Especially considering the age it should be given a thorough examination to insure nothing has been done to "Take it Out of Proof" as bore honing, chamber lengthening etc. Also note if this gun has a 3-ton stamp upon it, that was not put there 120 yrs ago. The 3-ton mark was not introduced until around 1954, I believe. In view of both old metal & wood one certainly does not have to run "Max" allowable loads. 7/8-1oz of shot at around 1100fps can be very effective loads. Loads of this nature are readily available with rather fast powders @ about 6,000 psi. These are my preferred loads for guns of this type. I personally will not use slow powders at very low pressures, except black.
