How To modify an airsoft shotgun to shoot multiple bbs?

Yeah, never try to modify a gun unless you really know what you're doing. If anything were to happen to the gun, you can't get your money back as the warranty would be null and void. You can pump it 2 or more times to shoot multiple bb's but they are very inaccurate and reduce the velocity by a lot. If you really want a multishot shot gun, try buying a UTG Multishot. They're pretty cheap and work better than a makeshift mulitshot.