After collecting enough of the blue components throughout each of the many levels, Stewie's gun will upgrade. The following list shows you what to expect with the corresponding number of components.
20 Components Ray Gun/Spread Shot Lvl 1
100 Components Plasma Ball Lvl 1/Spread Shot Lvl
200 Components: Plasma Ball Lvl 2/Plasma Artillery Lvl 1
300 Components: Hyper Plasma Ball Lvl 1/Plasma Artillery Lvl 2
400 Components Hyper Plasma Ball Lvl 2/Shock Wave Lvl 1
500 Components Rocket Launcher Lvl 1/Shock Wave Lvl 2
600 Components Rocket Launcher Lvl 2/ Heat Seekers