There are a lot of ways to make a gun out of Legos. Some have triggers, some don't. Some fire blocks, some rubber bands, but almost all use rubber band in the firing mechanism. This video is a demonstration of building one such gun. This one uses a rubber band to launch Lego blocks. It is relatively simple and very effective.
Start your career in Graphic Design with the WonderHowTo's Beginners’s Guide to Photoshop Course