A lot of mods require the cutting of a barrel, a cap or doing something wich you can’t do with your bare hands. That’s why you need tools like a hacksaw, a knife, cutting pliers or even a chopstick.
Hacksaw: Is often used to cut hard barrels like the HGG ones. Quite easy to do and you get a straight cut, you can use a serrated razor or any other kind of saw resembling tool. Wrapping some tape around and cutting along it’s edge is a good idea.
Cutting Pliers/Wire Cutter: Cutting pliers are mainly used to cut of clips, small parts sticking out; and G-2 (or other soft) barrels. Wrap some tape around the barrel, and then just follow the edge of the tape and cut straight through. Afterwards you’ll have to sand it down a bit to get rid of all the sharp edges. The video below is made by DarkArcher.