To attach pieces probably use super glue or epoxy. I have an SLA printer so I can't be sure, but I think for FDMs, the piston shaft needs supports and the caps need supports. The assembly is pretty giveaway, but for clarity put the piston handle through the object inscribed with "Cap 1". Then glue the handle to the object with one hole and no text. When that dries, glue "Cap 1" to the big cylinder. It might be better to add grease at this point, to the piston. That said and done, glue the cap with 5 holes onto the rest of the body to cover the cavity. Now the water gun is done! go have some wet fun outside unless you're something like me and allergic to basically everything. :(