Malapterus wrote:

I have tried and failed to find a standard unit of measure for the explosive potential of a material. Can anyone help me?

I am basically trying to compare different explosives and combustibles by volume.

The end goal is to compare them as the propellant in a rifle cartridge. I know there are more things than sheer power to consider; such as the speed of combustion, how sturdy if a chamber you'd need, and what it takes to combust the material.

An equal volume of plutonium would definitely have more energy than black powder, but you'd need a hell of a chamber, a crazy mechanism to ignite it, and I have no idea how fast it burns so you may need an excessively long barrel.

Any help with my math?

The standard measure for explosive energy is the "gram of TNT equivalent", which is exactly one Cal (or kilocal) of energy. If you want how much per unit volume, then you want a reasonably volume unit like a liter. Then you find the energy per unit mass and the density.

Energy/Mass * Mass/volume = Energy/volume. Your units of Energy are food Calories (i.e 4184 Joules each), your mass units are probably grams, and density standard units are grams/milliliter (1g/mL is identical to the standard temperature and pressure of water, most other density units are equivalent).

Looking up the density of TNT I get 1.65 g/cm³ (which is the same as kg/L or g/mL), so TNT is 1,650 Cal/liter.

Wikipedia claims that modern black powder liberates 3 megajoules per kilogram and is 1.7 g/cm³, so 3,000,000 J/Kg * 1.7 Kg/L / (4184 J/Cal) = 1,219 Cal/liter.

You can't get small amounts of Plutonium 239 to detonate in a nuclear chain reaction, you need [CLASSIFIED] Kg to make that work, but if you could, each atom releases about 211.5 MeV of energy.

Density is 16 Kg/L, 239 AMU is 3.96869e-25 Kg, 211.5 MeV is 3.3886034e-11 J. Combining: 16 Kg/L / 3.96869e-25 Kg/atom * 3.3886034e-11 J/Atom / 4184 Cal/J = 326,514,048,754 Cal/liter. Note that you need a lot of additional hardware to detonate the Pu239, so your actual force per bomb size will be much smaller.

Edited to add: And in real life, you'll never manage to get the Pu239 to all react at once. There's quite a bit of wastage, it's just that you're starting with 200 million or so times the energy density that makes it impressive.