Barrel!!!!!!!!!!!. Stock barrel really sucks. Expansion chamber( if using co2< helps it expand>), Air upgrade if possible, Electronic hopper.
Light (somewhat), not super fast but fast enough, accurate
bolts can break if not oiled really well, Wont re-cock sometimes.
I have played with this gun stock for a couple months and I didnt like it. I got everything you seen above and it is great. Not super fast but it works. Im thinking of getting an e-bladed cocker. If your looking for something simplem cheat, and very customizable, this is probably the way to go.
All in all its a good gun for beginners but if your looking for really high performance, go with a high end gun, but be ready to pay alot( over $300)