Place the chuck roast in a very large pot and cover with water. The water level must be at least 2 inches above the top surface of the roast. If you don't have a large enough pan a large soup pot will work, but the water level fills the pan over three-fourths full, and you will have lots of splatters on your stovetop (to alleviate this problem, cover your entire stovetop, except for the heating element you have your pan sitting on, with aluminum foil). It's imperative that you always keep the water level at least 2 inches above the roast. If you use a soup pot, add more water about every 30 minutes! When I cook the roast I USE A LID to partially cover the pot.
Bring the water to a boil, and cook for about 3 hours until the meat is tender. (I keep the heat a little lower than medium-high, which is a fairly strong boil. Recently I cooked a chuck roast weighing 4.16 pounds and it took 3 hour and 15 minutes, I'm sure the cooking time will vary according to the weight of the meat and how hard the meat boils).
Meanwhile, make the barbecue sauce. BARBECUE SAUCE INSTRUCTIONS: Into a LARGE saucepan thoroughly combine the ketchup, Worcestershire sauce, pepper, chili powder, vinegar, sugar, and onion. Cover and refrigerate for later use. Don't use a medium saucepan because the sauce gets so hot that it can pop out of the pan and can burn your hand.
When the roast is done, remove it from the water with a sharp meat fork and place it on a platter to cool slightly.
When the roast is cool enough to handle, shred the meat by hand (I shred the beef with a dinner fork, and then pull the larger pieces of meat apart with my fingers).
Remove the barbecue sauce from the refrigerator, and over high heat, stirring constantly, bring the sauce to a boil. Remove from heat and cool.
Pour the sauce over the meat, and refrigerate for at least 8 hours before serving in order for the sauce flavors to absorb into the meat.
When ready to serve, reheat the meat on the stove, in the microwave, or in an electric slow cooker.
Serve on buns.
Note: I freeze any meat that I don't use up within 3 to 4 days. I use fold-top plastic sandwich bags, placing 1/2 cup barbecue beef in each bag. I then, place all the little bags within a large freezer bag and place them in the freezer. One 4.16 pound beef chuck roast will give you approximately 15 servings (1/2 cup each).