If you ascertain their start position,say x=0, y=0, z=0 then you can add a random value to each axis (say between -10 and +10) and set the location to the new x,y,z. Start each random from the original origin and you'll get your guns moving around a cube shape where the original position is in the center.
Another way to keep the gun on the edge of a sphere shape would be to make the gun a child of a parent actor and move the child out by the max distance you want. Then rotate the parent randomly and keep the child always facing forward using clamp. Now your gun will randomise around the sphere (but not positions inside the sphere)
Hope these help.