A few months ago I posted here about my Charles Smith 13 bore double rifle. On the bottom of the barrels it is engraved "2 3/4 DRMS powder spherical ball". This always puzzled me as it is a relatively heavy gun and the rifling twist is about 1:100 which should be perfect for heavy charges. It shot quite well with 75 to 90 grains of powder. I didn't try much more than that out of respect for what was engraved on the barrels.

So on a another forum a knowledgeable fellow that some of you know (pukka) was talking about this very subject and explaining the difference between drams (27 grains) and drachms (60 grains). Apparently powder was measured in drachms back in those days and the two were often confused until the Weights and Measures Act of 1878.

So rather than 75 grains, my rifle may have been regulated for 155 grains which would make more sense. I'm pretty knowledgeable about this stuff (I thought) but this is the first I have heard of this.
Any thoughts?