Again, it may not be linear but Kynoch says the 416 Rigby is at 18 tons with 410gr at 2300+fps.

A loading site lists a similar weigh/velocity with ??powder at 42,000 cup which is of course not equivilent to psi.

Looks like 42,000 may be the max cup pressure for 18 tons or does the tonnage also vary with the diameter/surface area of the hull enclosedby the barrel??

Any engineers or gee whiz guys out there can sing in anytime!!