This question appears to fit comfortably into no subforum. But since I ask about black powder and a short cylindrical shape Black Powder Cartridge Rifles seems to be closer than any other. How much FFFg powder can be put into a .310-inch cylinder that is .125-inch deep?

The volume of your cylinder is pi-R-squared-times-depth = 3.1416 x .155 x .155 x .125 = 0.0094346 cubic inch

From the internet: FFFg Goex density = 238 grains per cubic inch, trusted but not verified

238 x .0094346 = 2.25 grains of powder

Is that for a .31-cal wildcat miniature BPCR?

Very miniature.
You can compress black powder you know... And it burns better if you do.
What are you doing; this can't be a cartridge, obviously.

Doing a tiny cylinder like this, an eighth-inch long 31-caliber volume, suggests what difference a longer neck might make on a cartridge ...
But BP and only .31-caliber?
The suspense is killing me.

It sounds like a gas check for a lead bullet.

Yeah, inquiring minds want to know,
But I can tell you that, this much FFFG is not enough to blow the brains out of a gnat.