There's some hex editing you can do. Like each gun is made of parts and such, so each part contributes to a stat. For example, each weapon company has about five parts (i.e. barrel, stock, sight) for each type of gun it produces. The game just randomly slaps parts together when you open a chest. However the people edit them, they usually just slap all the 'best' parts together, then edit each value, like clip size, accuracy, damage, etc.
Just what I think. I don't really support modding weapons, since this is an online game. I'm fine with experimentation, but keep it out of multiplayer.