How can I build a human-scale Lego gun?
There are some Books available that include building-instructions (i don't own one of these, but i would choose the most current):


The BrickGun Book (Book released in 2013, site has occasional updates as of December 2018 with more instructions)
Badass LEGO Guns (December 2010)
Weapons for LEGO Lovers (February 2009)
as Joubarc said the at least 4 years old Forbidden LEGO (July 2007)


to answer your question about magazines/reloading mechanisms: see this youtube-video that shows such a mechanism in detail (although it's a bit hard to perceive details).

in addition the book about LEGO Technic Simple Mechanics might also be interesting for you, if you want to try building a own gun instead of just "copying" from a detailed instruction.