Most wheel bearings are not set up to use a grease gun. They have to be removed, cleaned, inspected, and packed with grease. If you are not mechanically inclined, you should let a professional do it. By your user name, do you have a connection to Harvard University? Our son got his Masters there, and although I'm a retired truck mechanic, he never picked up any mechanical knowledge from me and would have no idea how to do wheel bearings. We all have our own area of expertise, but certainly capable of learning other tasks. Our son calls me with mechanical questions, I call him with computer questions. My opinion is that if you wish to do your own repairs, have a knowledgeable person show you, don't always rely on something you find on the internet. Some of the info I've found online just scares me the way people do things.