Cook the rice. Rinse the uncooked sushi rice in a fine-mesh strainer under cold running water until the water runs clear. In a medium saucepan, combine the rinsed rice with 1 1/4 cups water and bring to a boil over high heat. Reduce the heat to low, cover the saucepan with a tight-fitting lid, and simmer for 18-20 minutes or until the rice is tender and the water is absorbed.
In a small bowl, combine the rice vinegar, sugar, and salt. Mix until the sugar dissolves. Stir the mixture into the cooked sushi rice until well combined. Set aside to cool.
In another bowl, mix the diced tuna, mayonnaise, and sriracha sauce until well combined. Set aside.
Lay a sheet of nori seaweed on a clean surface, shiny side down. Use wet hands to spread half of the prepared sushi rice evenly over the nori, leaving a small border around the edges. Place half of the spicy tuna mixture, sliced avocado, cucumber, and carrot on the rice in a line across the center of the nori.
Roll the nori tightly around the filling, starting at the bottom and rolling up to form a cone shape. Use a small amount of water to seal the edges of the nori, and repeat the process with the second sheet of nori and remaining ingredients. Enjoy with soy sauce and wasabi!