Originally Posted by john traveler

The usual format is drams (equivalent), shot weight, shot size.

Your 2 1/2-1-6 would read as: 2 1/2 dram equivalent of powder, 1 ounce of number 6 shot. A field load suitable for rabbits and squirrels.

A more powerful load for ducks and pheasants would be 3-1 1/8-4: 3 drams equivalent of powder, 1 1/8 oz of number 4 shot. This would give a higher velocity with a heavier load of larger shot.