These physics equations are easy, might actually try them. LOL or not:
http://www.1728.org/energy.htmUsing a weight of 8 grams (for a 9mm bullet), with a speed of about 340m/s as it leaves the gun, that gives it ~460 joules of energy. That 460 joules of energy is enough to push a 75kg person back at 12km/h (a bit faster than average walking speed). You could say the arm will absorb some of the shock but there is no ground to shift that shock into, so you will take all that 460 joules as kinetic energy, however some of it will likely cause you to spin so the final speed will be lower.
Another way to think of it is if you burnt 1kg of wood, one inside a large bottle, another in a huge room. The same amount of energy is released in both cases, but the smaller area has the energy more concentrated, so the bottle will end up much hotter than the large room. Same why the bullet will go faster than you with the same energy.
Gunpowder doesn't need air to burn, it already has a very strong oxidant in it. The activation energy to trigger the burning reaction is fairly low, so don't throw bullets hard or drop kegs of gunpowder lol.
Basic gunpowder formula and reaction is
2 KNO3 + S + 3 C → K2S + N2 + 3 CO2.
Basically this is two parts potassium nitrate, one part sulphur and three parts pure carbon (from charcoal). It burns into potassium sulfide, nitrogen gas and carbon dioxide.
Also hello NSA spies, who are probably reading this now it mentions gunpowder formula, guns, space, air's mum, packages and who knows what other key words