[Skepolitic]

One of the fundamental laws of nature is conservation of momentum. So your velocity depends on the ratio of the inertial mass of your body, spacesuit, and 45 pistol to the inertial mass of the 45 bullet.

The conservation law is m1*v1=-m2*v2. Rearranging terms,

v1 = -m2*v2 / m1

where v1=your velocity
m1=inertial mass of you+spacesuit+pistol
m2=inertial mass of 45 bullet
v2=muzzle velocity of the 45 bullet

[Sigurdrifta]

You also have to take into account the gases (and particles) ejected from the muzzle.