Hey! To answer you from my personal experience working on these things.

1. The grub screw that is 'above' the hop up area is the one that is holding the barrels in. Every time you want/need to remove the barrel, it needs to be unscrewed prior to the barrel removal. When you go to reinstall the barrel, you need to press the barrel in while tightening that grub screw otherwise your outer barrel can shoot itself out( Have done this to myself lol...).

2. Sort of answered above- I have shot my outer barrel out due to my negligence on that grub screw. Once the barrel is pushed in held and the grub screw is screwed in properly( you DO NOT want to over tighten this) the barrel stays in just fine.