Is there a formula for converting volume to weight in black powder ?
Black powder is usually measured in "grains". Grains is a unit of weight measurement. 1 grain = 1/7000th of a pound. Conversely, 1 pound contains 7000 grains.
Powder can be
approximately measured out
by volume. The trick to using a volumetric measure is to use it the same way every time, so that you end up with the same amount every time. For example, if you fill a brass cartridge case with powder and then tap it on the table, it will settle and you can fit a little more powder in it. Thus you will get different amounts of powder if sometimes you let it settle and other times you do not.
If you want the ultimate in precision, such as in competition shooting, then you will want to control all variables that you can. This means weighing out your bullets and eliminating any that fall outside of an acceptable range, and weighing out your charges.
I use a Lee Precision powder measure - it is a volumetric measure. I will adjust it until it throws the charge I want - say 45 grains of 3F powder, as verified with my Chargemaster 1500 scale. Once I get the Lee Precision powder measure dropping what I want, it will generally drop within .2 grains.
So what I do is set a little flat-bottom plastic tube on my scale and zero the scale. Then I drop the charge from the Lee Precision powder measure into the tube and set it on the scale to confirm the weight dropped. In this way I drop powder by volume but check each charge by weight. The Lee Precision is so precise that I really don't need to do this but every once and a while I will get a charge that varies by .4 grains or so so I dump it back in the hopper and dispense again.
These things may be overkill but like I said for competition shooting I try to eliminate all variables. I have taken home medals at every regional skirmish I have ever been to in the 7 years I have been shooting N-SSA competition.
Steve