From what my research has yielded, the dual stage propellant is what's used in solid projectile type 40mm shorts. The special double propellant charge and the short shell is what makes it possible to fire one of those big heavy HE warheads for such a long distance and still be a shoulder-fired weapon.That's certainly true of stuff like explosive warheads and flares. But 40mm canister rounds (stuff like shot or flechettes) are pretty much the same as a massively sized-up shotgun shell. That's why I chose to add recipes for those two types exclusively; making them is mechanically treated like making big shotgun shells of the same type with a higher difficulty and more resources per-shot because they are so very similar.

However, if you think that it should be a separate ammo type, I can do that as well. I was just trying to avoid doing that too frequently with reloads, or else one of these days I'm going to end up going through and making reloaded versions of all the compatible ammunition types.