Divide ground sirloin into eight 1/2" balls and eight patties. Place patties on grill and add sauce on each pattie, grill for a couple of minutes turn over add more sauce. Put one ground sirloin ball into each hole in pineapple slices. Place on grill, add sauce, and grill a couple minutes more before turning over and adding more sauce. Place buns on grill to warm. Place one pattie and one pineapple slice between two buns and serve.
Sauce: in saucepan over medium heat, add ketchup, barbeque sauce and pineapple juice and bring to boil.