A handgun round can knock someone down and not knock the person firing the gun down. Momentum is not the only force involved in a situation like this. if i throw a brick at your head as hard as i can its not going to knock me down but the "shock force" of the impact can still knock you down. With ammo that expands rapidly on impact you have "shock forces" as well as Cavitation that increase the energy that is transfered into the object being hit. Look at the effect this HP round has on the ballistic Gel in this video. the expanding pocket in the gel is what is known as cavitation and it increases the the force in front of and surrounding the round. YouTube - Hollow Point Bullet Through Gelatine
A handgun round cannot knock someone down. A handgun round may cause someone to fall down, from a variety of intermediate causes, but the bullet has not pushed the person to the ground.