exactly - Forsythe did note that many sportsmen simply loaded their "fast twist rifles" with large charges and used them as if they were smooth bores, simply to gain the power needed for dangerous animals.
Forsythe also noted that he himself has such a fast twist rifle, a 13 bore with a 3 foot twist that stripped if he "gave it more than 1 1/2 Drachms of powder" - and that with that charge, it's trajectory was 13" over 100yards- which possess absolutely no point blank range.
Thus, we know although he used the word Drachms he actually meant Drams. A Dram carryed 1/16th ounce or 27.34gr. weight of powder, whereas a Drachm is actually an Apothecary's measurement equaling 60gr. weight. Thus 1 1/2 Drams, ie: 40gr. of powder would indeed give about a 13" trajectory and would not be a hunting charge, for anything other then bunnies. (it works for them BTW - even in an 11 bore, whereas I used 30gr., ie: 1.1 Drams in my 14 bore for them)
In the same Tome, a quote of Samual Baker's referred to a hardened 14 bore ball using 4 1/2 to even 5 Drachms of powder. He most certainly was talking Drams as well, not a real Drachm. The resulting 5 Drachms wound have been 300gr. of powder, which was actually a 6 or heavy 8 bore charge, not a 14 bore charge.
This is 5.13 drams with a 9 1/2 pound rifle.