Rinse sushi rice until water runs clear completely.
Cook rice according to package directions until tender.
Mix vinegar, sugar, and salt in small bowl.
Add vinegar mixture to warm rice gently.
Cool rice to room temperature before using.
Boil shrimp for 3-4 minutes until pink.
Drain shrimp and cool in ice water.
Chop cooked shrimp into bite-sized pieces.
Combine mayo, sriracha, sesame oil, and lime.
Mix sauce with chopped shrimp until coated.
Place ring mold on serving plate carefully.
Press rice layer firmly into mold bottom.
Add cucumber layer over rice evenly.
Spoon spicy shrimp mixture on top.
Add avocado pieces as final layer.
Remove mold gently to reveal stack.
Garnish with sesame seeds and nori.