As I understand it, it is classified as a handgun accessory, so on the Fed level no problem. State and Local laws are what you would need to be concerned with.
Their own web site points out that in California it is only legal if you don't have the handgun for it. If you do then it falls under the CA assault rifle laws.
Just glanced at the HI statutes, looks like it might fall under "assault pistol" since it allows holding the barrel with a second hand.