I'll ask my son tonight, he's into that stuff. But my sense of it is that by design they won't kick much, if at all. The mass of the pellets just isn't that much and the volume of air in the ones that are not spring-powered isn't that great compared tot he interior volume of the barrel. This is why you use *actors*:-) They can mime some kick-back, but of course this is of litle use after the fact.
Still, using a compositor you couldf ake a little bit of kick by isolating the gun area and applying some "earthquake" motion to it. Might need to roto it, depending on the shot.
Airsoft guns dont kick at all and you'd still have to add a blast from the barrel. A real hand gun with blanks is best. Just cap it at the end. You'll find of you just paint it ultimatte green you can key it out with no problemos at all.