Great questions. The first is metaphorical. I'm not sure about the context, but it would usually be used to show happiness. If someone is very happy, they feel light, and want to jump and dance. They feel like they are "floating on air." By contrast, if they feel sad, they feel slow and heavy. They are "weighed down by their problems"; they "have a monkey on their back"; they "are carrying a burden". So, if someone walks or dances happily, they "float". "To barrel" is to move like a gigantic, rolling barrel. Imagine a 5,000-pound barrel rolling toward you. You wouldn't try to fight it--you would just run away. If a bull chases you, it "comes barrelling toward you". "To barrel" isn't just about speed, it's about momentum--a ballet dancer might be able to run very fast, but it would be very difficult for her to "barrel" at anyone, because she wouldn't have enough mass to act like a barrel, bull, or car.