Place the rice in a fine mesh strainer and rinse until the water runs clear.
Put the rice in a rice cooker with the water and cook it.
In a bowl, mix the sugar with the mirin, rice vinegar, and salt.
Let the rice cool slightly, uncovered.
Gradually add the vinegar mixture to the rice, gently mixing.
In a bowl, flake the tuna with a fork.
Add the Japanese mayonnaise and sriracha sauce, mixing well.
Place some rice at the bottom of an onigiri mold. Create a cavity in the center.
Place about 0.7 oz of tuna in the cavity.
Cover with more rice.
Press and unmold the onigiri.
Cut the nori sheets in half.
Wrap each onigiri with a half sheet of nori.