It's always a good idea to rough things up where glue is concerned, especially if it's bonding to smooth surfaces like plastic. 80 grit sandpaper is good for this or just scraping with an xacto blade..
liquid epoxy + rough on all surfaces = a relatively permanent bond. Tho I should note most epoxy doesn't bond well to vinyls, (i.e. pvc) and is only passable on ethylene plastics (i.e. 2 liters.) I'm confident that a squirt gun would be made of styrene.