Preheat oven to 350°. Slice sub rolls in half and place on a baking sheet. Spread 1 to 1/12 tablespoons of mayo on each half, according to your preference. Divide the roast beef and layer on one half of each split sub roll, then top with your favorite barbecue sauce to taste (I like about 1/3 cup for two subs when I make this). Cover the bbq sauce and beef with two slices of cheese per sub, and insert the sandwiches open faced into the oven. Bake for 5-7 minutes, until cheese is melted and beef is warm.