Measuring Black Powder: Weight or Volume?

Most people today measure black powder by volume using any one of the various powder measures on the market, and if you ask the community in general whether it is possible to measure powder by weight, many people will run off on extremely vehement rants about how all powder calculations are done by volume and tell you that you can’t go by weight or you will blow up your piece.  They are partly correct, but lack the full picture; you absolutely can measure powder by weight, but doing so requires some information and a bit of elementary arithmetic.

Black powders, including powder substitutes such as Pyrodex, all have different densities.  Load tables are almost always written for volumetric measurements because most powders are formulated to have similar (not identical) chamber pressures for equivalent volumetric loads of a given granulation.  Thus, 30 grains of Goex 3F powder, measured by volume, creates roughly the same amount of chamber pressure as does, for example, 30 grains of Swiss No. 2 or 30 grains of Pyrodex P when they are measured by volume, too.

But that 30 grains marked on the powder measure is not actually a measure of volume; a grain is actually a measure of weight using the avoirdupois system in which there are 7,000 grains to the pound.  Most powders, however, are formulated so that the same volume of powder will create roughly the same amount of chamber pressure as most other powders (note the careful use of “most” and “roughly” here!).  As a result, most shooters use volumetric measures which have been calculated such that the mark on the powder measure equates to the hypothetical norm of volume necessary to create a given amount of chamber pressure (approximately 0.07 cc’s/grain).

This is extremely convenient because it allows shooters to use the same powder measure for almost any kind of black powder they might use, but it can lead to a dangerous misunderstanding.  Let’s consider a hypothetical shooter who decides he wants to use an odd amount of powder (e.g., 32 grains) when he’s working up a load for his piece, but his powder measure is only calibrated in 5-grain increments.  If he simply weighs 32 grains of powder without considering the type and granulation of powder he’s using, he can end up with far too much powder for safety.

Examine the chart below which has been excerpted from this table:
<http://www.curtrich.com/BPConversionSheet.htm>

Vol.     Vol.     Goex 3F          Goex 2F          Pyrodex P
CCs     Grains
0.67     10        10.1                 10.5                 7.6

From this, we can see that 10 grains of Goex 3F, measured by volume, weighs 10.1 grains by weight, while that same measure of Pyrodex P weighs only 7.6 grains by weight.  Thus, 30 grains by volume of Goex 3F actually weighs 30.3 grains, and 30 grains by volume of Pyrodex P actually weighs 22.8 grains because it’s so much less dense.

Using the afore-threatened arithmetic, if our hypothetical shooter decides he wants to use 32 grains of Pyrodex and measures it on a reloading scale instead of using his powder measure, he’s actually going to get a charge equivalent to 42.4 grains of Goex 3F, which would create almost 15% more “bang” for the load.

However, we can still find a 32-grain load for our Pyrodex shooter if he knows all of the information presented here.  Looking at the table linked above, Pyrodex P weighs approximately 76% of what Goex 3F does, so with more simple arithmetic (or just reading the table), we can see that to make a 32-grain load equivalent of Pyrodex our shooter should weigh out 24.3 grains on his scale.

Another consideration with regards to weight versus volume is that volumetric measures are neither very accurate nor very precise.  With very light loads (e.g., pistol loads) the differences can be fairly small, but for rifle loads they can be more significant.  When I measure 65 grains in my volumetric measure the weight can vary by 5% or more, but on average a 65-grain charge of 2F powder measured by volume with my powder measure comes to approximately 61.4 grains on my scale, and 3-5 grains can make a difference.

Moreover, each granulation of powder takes up a different amount of volume because the particles of the powder are of different sizes.  Thus, 10 grains of 3F powder, measured by volume, will actually contain more powder than 10 grains of 2F powder, which is why most people use the rule of thumb which says that when shooting 3F you should use 5% less Goex than you would if shooting 2F or 10% less if shooting 1F.

As this essay demonstrates, black powder can be measured by either by weight or by volume, but doing so requires an understanding of how such measurements actually work and what they mean for each brand, and for each granulation of each brand, of powder.