Just a bow to gun texture pack, simple. The arrows are changed to bullets, and tell me if something goes wrong. Be the way the green bar is the power chamber, if it is full, its full power. The skeleton is "Corrupted Steve," and he drops bones as his spine and ammo as his leftover ammo when killed. The glass is clear so you can spot monsters through windows and snipe them. The idea of the clear glass was from WoodCraft2, as he said it in the comments!
Another idea by papado123 was to make creepers steve with tnt strapped on their backs!

HD version : http://www.planetminecraft.com/texture_pack/bows-to-guns-pack-hd/