Volume or weight
If you are using anything other than real BP, don't go by weight.
The substitutes, like Pyrodex, are meant to be an equivalent by volume, not weight.
If you switch to real BP, then by all means weigh your charges if you want to be more consistent.
If you want extra consistency with Pyro, then measure out whatever charge seems to work best in your gun by volume, then weigh it, and use that weight.