Well, the guns won't exactly be flat shooting. The gravitational pull of the earth is about the same as if you were standing on the earth. Orbits are pretty neat. You are going so fast that your plummet to Earth....misses.
Vacuum won't be a problem for unseating bullets.
Force = Pressure x Area
Pressure = 14.7 psi at sea level
Area = pi*radius^2
Area of a .45 caliber bullet = 3.1416*(.452/2)^2 = .159 in^2
force on a bullet caused by air pressure in the cartridge = 2.33 lbs
the ammo probably doesn't need a crimp.
Shooting the gun will change your current orbit slighty because of the change of velocity due to Newton's laws.
As far as the bullet hitting you, I'm going to have to sharpen up my partial differential equations, but the bullet should be in a much different orbit, because of the difference in velocity.
Aiming over any great distance (like a mile) will require a heck of a computer, because the trajectories will be anything but flat in the Earth moon system.
