THIS is very telling.

Although it is obvious Col. Hawker was talking about using a 60gr. drachm, as 1 1/2 would be 90gr. = to 3 1/4 drams, typical for 1 1/2oz shot in a ML.

By the 1850's, Samuel Baker(of the Nile) and Forsyth both mixed the terms, dram and drachm.

I am VERY sure Forsyth was not talking about using 5 drachms of powder (300gr. of #6 C&H) in a 9 pound double 14 bore rifle, although he used the "drachms" term.

I accidentally fired 330gr. in my 9 pound 14 bore single rifle with a 484gr. ball. I had double loaded my 6 dram charge (likely talking), but with one ball only. That "shot" was memorable.

I was sitting on a stool shooting over the chronograph at the time, on the jail range, teaching one of the guards how to load his new muzzleloader - LOL. The shot lifted me up to the standing position. It was a good thing my feet were slightly behind my knees and on my toes, or I'd have gone over backwards. Strangest thing - BOOM! and I was standing up.
The chronograph indicated 1,770fps for that shot.

Earlier testing indicated 190gr.2F (7drams) was the likely max. efficiency for speed, as that charge produced 1,700fps,
which was 200fps. higher than 165gr. 200fps for 35gr. more powder is not bad, but 70fps for another 130gr. is horrid.