There is no expungement of federal crimes. There is a pardon process. Here is a really good web-site with a lot of very understandable information about what it takes to get a pardon for federal crimes: http://answers.google.com/answers/threadview/id/478249.html
If your conviction is for a U.S. military offense, you would have to first apply for clemency with the branch of the military which issued the conviction before invoking the pardon process.
My responses to questions on Avvo are never intended as legal advice and must not be relied upon as legal advice. I give legal advice only in the course of an attorney-client relationship. Exchange of information through Avvo's Questions forum does not establish an attorney-client relationship with me. That relationship is established only by individual consultation and execution of a written agreement for legal services.
Thanks for your posting.
The answer to your question is that, as others have noted, there is no expungement statute for federal crimes, so your only option is to obtain a presidential pardon.
I hope that this helps, but please feel free to ask if you have any further questions. It's my pleasure to help in any way that I can. All the best to you.