On the manual, there seems to be an additional feature listed beside making you shrink and unshrink.

If you twist the muzzle in a certain way to the right, the green hue of the raygun changes from green to blue, firing a blue light and summoning a portal. 

Then out of nowhere, a screen pops up from the gun, allowing the user to type in any world, no matter the time or reality they wish to go. At least according to the instruction

Now things got more interesting, but where to go first...