ATF Online - Bureau of Alcohol, Tobacco, Firearms and Explosives
You can buy anything in any state as long as the gun is delivered to an FFL in your state for you to complete a 4473 and take delivery in compliance with your state laws, such as waiting periods].
If the states have passed a law to allow residents to purchase out of state and take possession out of state, it is limited to rifles and shotguns and both states must have an agreement. So, a resident of Kansas can buy a gun and take delivery in OK,MO, NB, CO, completing a 4473 with teh selling dealer.
Handguns must be delivered to the state of residence, to an FFL.
Not sure, have not checked whether the federal law now allows long guns to be purchased in any state or only states with common borders.