federally speaking you wont have an issue crossing state lines with them, nor needing a tax stamp. a brace does not make a pistol a rifle, at least according to *CURRENT* batfe regulations anyway, no matter the manner in which its used. that of course is subject to change on a relative whim, hence this thread.
you'd have to check your state specific hunting regs to see if theres anything funky that it would interfere with, etc.