A DuPont Ballistic Table published in Parker Brothers' “The Small Bore Shotgun” c. 1920 listed 16 gauge:
2 3/4 Dr. Eq. 7/8 oz. = 7,035 psi
3 Dr. Eq. 1 oz. = 1275 fps = 8,980 psi
using Bulk Smokeless; Dense Smokeless would be 1000-1500 psi higher
http://parkerguns.org/pages/PDF%20Do...%20Shotgun.pdf
Your gun can not be presumed to be safe with ANY load until you are able to answer Dean's question - what is the end-of-chamber, forcing cone, 9", and minimum wall thickness and where?
And are the barrels fluid steel or Damascus?