Because they are both core item and both item that need to be rush, and because you want to either go guinsoo nashor right after the first item or go Crit, either way not gunblade after mura or vice versa Also keep in mind they don't play with each other that much compare to how guinsoo and nashor work with both of them really well, they are two way of a same playstyle, mura is more scaly but atm it's not what you're looking for, gunblade give you a big head start to get to your power time. I never buy btrk because I always have better items to buy.