You can do this by having your gun, in addition to firing your bullet, also fire a no-colision plane. this plane scales from 0 to 100 in 100 frames. the gun also shoots a bullet that travels one frame every logic tick. when the animation of your laser is running at one frame for every logic tick, the end result will be a no-colision plane filling the space between the gun and the bullet before impact. In the demo below, I have the bullet invisible. All you can see is the scaling plane.
To make your plane scale to get it elongating at a rate of one Blender unit per Logic Tick, use this logic setup.
You need to start with a plane, and move the origin to one edge. then move the edge opposite the origin in to make it One blender unit by Two blender units.
Then set the timeline to frame 100, and in object mode press S to start the scale, press X to constrain the scale along the X axis. Press 100 and then press Enter this will set the scale so that the demotions will be 100 blender units by 2 blender units. press I to set a keyframe, and select Scale
this will set a keyframe with the scale on frame 100.
Now do the same thing at frame 0, only this time we need to set the scale along the X axis to 0.
Set the timeline to frame 0, press S to start the scale, press X to constrain the scale along the X axis. Press 0 and then press Enter this will set the scale so that the demotions will be 0 blender units by 2 blender units. press I to set a keyframe, and select Scale
Now we have the scale animated from frame 0 to frame 100.
However, if we move the timeline, we can see that the animation is not quite right.
We can fix this by going into the graph editor and changing the curve type to linear
Now when we test it, we can see that every frame makes the plane scal one blender unit longer.