Alright here's what I would do:
Find the position of the mouse and the gun. Then find out distance horizontally and vertically from the gun to the mouse.
Use trigonometry to find the angle (tan theta = opp/adj therefore: theta = inverse tan(opp/adj)). Work out what velocity you want to give it, so that could be, say, 5. Take that as the hypotenuse.
So you can work out the x,y velocity by doing: 5 sin angle and 5 cos angle (angle is what you worked out before). Then just apply that amount every second or so.
I'm a bit hazy on my VB (especially since I learnt VB6) so I'm not even sure this is possible or not.