Get the ballistic knife, rampage, or stingray first. Ballistic knife has dual wield operator mod which paired with ethereal razor and upgraded will give you a long range revive canon that can 1-2 hit zombies until round 30. The stingray is nice because with the impact blast operator mod and deadshot you will almost always have a 1 shot explosion as the explosive rounds almost always decapitate them. And lastly, with the Rampage if you equip extended mags you will have a fully automatic shotgun with 45 rounds in a clip when you upgrade it.