We now have excellent data as to the pressures generated by the turn-of-the-century loads that were recommended by Parker Bros. and the other double gun makers
https://docs.google.com/document/d/1...UOZEFU/preview
Scroll down about 1/3 for Parker's proof testing protocol
A DuPont Ballistic Table was published in Parker Brothers' “The Small Bore Shotgun” c. 1920
http://parkerguns.org/pages/PDF%20Do...%20Shotgun.pdf
It is clear that this table converts Long Tons to PSI simply by multiplying by 2240; NOT using Burrard’s conversion
Numbers require adding 10 - 14% for modern piezo transducer pressures.
12 gauge
3 Dr. Eq. 1 1/8 oz. = 8,110 psi
3 1/4 Dr. Eq. 1 1/8 oz. = 8,960 psi
3 1/2 Dr. Eq. 1 1/4 oz. = 9,900 psi
16 gauge
2 3/4 Dr. Eq. 7/8 oz. = 7,035 psi
3 Dr. Eq. 1 oz. = 8,980 psi
20 gauge
2 1/2 Dr. Eq. 7/8 oz. =
12,655 psi
Pretty easy to match those numbers with factory loads today...NOT that 1 1/4 oz. 3 1/2 Dr.Eq. would be a good idea for 100 year old wood.
We also know from published pressure-distance curves that the pressure with modern 1 1/8 oz 1200 fps loads after 18" is < 500 psi
OTOH I am increasingly convinced that there are VERY few vintage doubles with unmolested barrels
and that "should be safe with light loads" is very dangerous advice
