The old 96:1 ratio of gun to shot weight is still a better determination for controlling recoil than the square load. With the square load in a 16 being only .79 oz the 96 ;1 ratio would give you a gun of 4 3/4 lbs. If one plans on using 1 oz loads the the Ratio gives a 6 lb gun weight, far more comfortable with continuous use of ounce loads.
I really failed to see anything at all in the above link which was much applicable to shotguns. Also it would seem that even Black Gunpowder improved a lot over the next decades of use. 1000 atmospheres (14,700 psi) was stated to be the maximum achievable pressure in a closed cell vessel. Some 100 years after this was written sporting shotguns were proofed at a higher pressure, with black gunpowder, than this & it wasn't done under a closed cell situation either.
PS; This has been an interesting thread, glad you posted your question Stan. I hope we answered it sufficiently to the fact the writer of the article just didn't word it good & that it should have applied to each component of the load & not the entire load
