Muskets aren't considered firearms right?
You could get a musket.
Or a sword.
For purposes of
certain laws, they are not. For example, the laws regarding sales across state lines.
For other laws, the terms are defined differently. For example, a "machinegun" is defined to include any device which can make a firearm fire more than one shot with a single pull of the trigger, even if that device would not meet the dictionary definition of "firearm." In once case, someone use a shoelace to give a trigger enough "spring" to activate itself due to recoil after the first shot. The Feds defined that shoelace, in and of itself, as a machinegun.
Basically, they can define anything any way they want it, in order to make the laws as confusing and entrapping as possible. Lawyers are really not much more than highly-paid translators who are fluent in legalese.
I wouldn't assume that any particular state's laws (and caselaw) would exempt muskets from the definition of firearms, for the purpose of the "no possession by prohibited persons" thing...
Joe