In a Dutch oven, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in the ketchup, water, brown sugar, vinegar, salt, mustard, chili powder and allspice. Bring to a boil. Reduce heat; simmer, uncovered, until heated through, 35-40 minutes. , Spoon about 1/2 cup meat mixture onto each bun.