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The word act, as used in the term Act of Congress, is a common, not a proper noun. The capitalization of the word act is deprecated by some dictionaries and usage authorities. Some writers, in particular the U.S. Code, capitalize Act.
So you would only capitalize federal when you use it in the name of a federal agency, an act, or some other proper noun. When writing about the Federal Government in its official capacity using the term as an official title, you should capitalize both Federal and Government. Most of the time, we use federal simply as a form of general classification, which would not require us to capitalize it.