Heat an outdoor grill to medium, about 400°F, with a direct-heat area and a cooler indirect area. Warm a 10-inch cast-iron skillet over the direct heat for 3 minutes.
Add beef, pork and onion and cook for 8 to 10 minutes, breaking the meat into crumbles, until both meats reach 160°F. Lift the pan off the heat and spoon away excess fat.
Stir in ketchup, molasses, vinegar, mustard, Worcestershire, paprika, salt, pepper and water. Move the skillet to indirect heat and close the grill lid.
Cook for 10 to 15 minutes, stirring every few minutes, until the sauce clings to the meat; lower the grill heat if the bottom starts sticking.
Toast the cut sides of the buns over direct heat for 1 to 2 minutes, divide the filling among them and serve.