In the 1850's-1860's - Drachms and Drams seemed to have been used interchangeably, which of course, lead to gross misunderstandings due to the gross difference in their weights.

You are likely correct with the apothocary's weight of 60gr. per drachm - if the gun is 9 pounds or over, rather than the 27.3gr. dram weight.

100" sounds like a typical twist for 4 to 7 drams of powder in a 9 1/2 or 10 pound rifle.