By flipping a switch on the barrel of his asault rifle, Karasuma can change the bullet type to Viper. Taking advantage of its controllable trajectory, Karasuma can cause the bullets to go around the protection and hit the opponent directly from multiple angles. Even if the target can block them, they will likely be unable to move freely, allowing an ally of Karasuma to finish them off.
Izumi is one of the only two Border agents who can create new patterns for Viper every time they use it, the other being the specialist Rei Nasu. He has used this skill to make his bullets fly complicated routes so as to prevent the opponent from figuring out where they will hit and to attack without disclosing his location by altering their paths out of sight.
Nasu is one of the only two Border agents who can create new trajectories for Viper every time they use it, the other being the Shooter of the top A-Rank unit Kōhei Izumi. Thanks to this skill, her attack patterns are countless regardless of her position in relation to the target, especially since she can bypass obstacles. She generally uses Viper in Full Attack. The unpredictability of her bullets makes the difficult to stop or evade, especially thanks to her ability to anticipate the movements of the opponent. Her tactics are extremely varied and involve feints, sudden changes of target, focused or scattered fire, and aiming at multiple enemies at the same time. She first used Viper in chapter 96 (episode 44 in the anime).
Birdcage (鳥籠,Torikago?): By using Viper both through the main and the sub, Nasu unleashes a multidirectional attack to simultaneously target all enemies within her sight or visible on Radar, proving to be a threat even when surrounded. Feinting this technique and then concentrating fire enables them to thwart possible countertactics.