If you attempt to bring items in to the country and don't declare them, then get caught, at best you'll have to pay duty and taxes on the items, at worst you'll be charged with smuggling, which is a federal crime.
Either way, if you get caught, you will be flagged and be prepared to be pulled over and searched every other time you cross.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa