I've never heard of the standard US musket load being 150 grains. That's a huge amount of powder and doesn't seem to match up to the original arsenal loaded .69 caliber cartridges I've seen in museums and collections.
During the Civil War IIRC the buck and ball cartridge that was issued in the millions used 65 to 75 grains of powder. It was measured volumetrically so it could vary somewhat.
That's apparently how we came up with the 70 grains of powder in the .50-70 and the .45-70 after the war.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.