This is one good thread, i used to carried a normal size backpak for few months,only kept few items inside,air pump,cable lock,multi tool set,bottle of Slime,sometimes a digital SLR or/and a Handycam camcorder,the pack you show in the first post in hanging on the side, it will be off balance as you ride,not a good idea,i have tried that. far as the handgun,you dont want to keep it inside that bag unless you have to, i will recommand a good hip holster for a quick draw.sweat will cause misfire and rust? this depends on many factors,my Glock 36 been riding in an inside waistband holster for many sweaty summers without rust but this is not the topic right?