In order to avoid unwanted weapons and increase the chance of handgun ammo drop, I had to do every chapter separately. Once I completed or failed a chapter, I did the last part of the last chapter of Jake & Sherry campaign (because I believe is the shortest) to start a new game +, I chose the option to start the campaign without the weapons obtained previously (and all the inventory such as ammo, grenades or healing), I quitted as soon as I could and then I chose the chapter I wanted to do. So, I start every chapter with the starter equipment.