How do you get a federal concealed weapons permit?
The short answer, based on what I think you're asking, is you can't, because no such thing exists. The longer answer is, the only way to be able carry everywhere in the U.S. is to become a commissioned officer through a federal agency such as the FBI, or to be a commissioned police officer, and carry under the HR218 law.
Can a concealed arms permit be obtained with a violent felony on their juvenile record in California?
It depends on where you are. In the U.S. each state sets its own rules. For instance, in Tennessee, there is no such thing as a weapons permit, it's a handgun carry permit, and applies to handguns only. Other than handguns, basically anyone can carry any legal weapon, but to carry a handgun, you must have the handgun permit. On the other hand, Florida issues a "concealed weapons permit" and it covers any legal weapon.