as long as it's not a dangerous item there is no need to declare anything when shipping inside the country.
when some nosy motherfucker at the PO asks what I'm shipping in a zip tied guncase, I always answer "R/C Helicopter"
seriously, even if I WAS shipping an RC copter and not assault rifles i'd use a guncase, waaay more secure than some cardboard box.
in 7 years of buying and selling I've never had a package break open. Just pack the fucking thing well, you pay or get paid hundreds of dollars per gun, no reason to cheap out on packaging.