*Thinks with malicious intent* *Researches* Actually apparently anti-matter, or rather annihilated matter, "only" has a TNT-Equivalence of roughly twenty-one-billion-to-one, or a ratio of one to two-point-one-to-ten-to-the-tenth-power (2.1e+10 or 21e+9) or twice as much as ten orders of magnitude, so, if I'm reading
this unit-conversion correctly, that means that a single
Microgram of Antimatter is equivalent to
twenty-one Kilograms of TNT.
Wait, no, twice that because the Matter that it's mutually annihilating with would also contribute. So forty-two Kilograms
which looks a bit more like this.
Theoretically it would be possible to fire a gun using, if I did the math on orders of magnitude and converting between Grams and Grains correctly, sixteen Picograms of Antimatter.
I'd figure out how much you would need for each piston-ignition of a internal-combustion-engine but figuring out the TNT-Equivalency of fuel, the amount of fuel injected into each piston for each combustion-cycle, and then converting that into the Metric System is being a bitch so I'm not going to do it.
Edit: based on that TNT-to-Antimatter ratio I found and the Joules produced in a engine, and how much of that is lost when converted to torque, and dividing that between several pistons I get a number somewhere between seven and zero-point-two Picograms, which is 1e-12 or Ten-to-the-negative-twelfth-power Grams, of Antimatter. That's very few Grams of fuel.
Edit: also I found
a unit-converter able to convert between Picograms and a unit used to measure the weight of individual atoms, in a way that
really seems like it counts neutrons worth of atomic weight, and from there was able to calculate that a Picogram of Carbon comes out to about fifty-billion individual atoms.
Now that's Carbon atoms and if you're using Anti-Protons instead of Anti-Particles, as you bloody-well should, that gets multiplied by twelve, or thereabouts if you want to be precise enough to account for a Electron's worth of mass, to get you something like three-hundred-billion Anti-Protons, paired with the same number of Protons for a total of six-hundred-billion Protons worth of particle/anti-particle annihilation, per piston-stroke.