Federal law allows for the unlicensed, private sale of firearms, as long as both individuals reside in the same state, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives. Private, interstate sales between two unlicensed individuals are illegal.
Interstate firearms sales must involve a Federal Firearms Licensee. The purchaser must either travel to an FFL in the seller’s state to pick up the firearm or have the firearm transferred to an FFL in their state to complete the transaction.
Heckler and Koch manufacture many variants of the MP5, some of which have barrels under 16 inches. Rifles and submachine guns with a barrel shorter than 16 inches have additional restrictions under federal law.
Rifles and shotguns may be transferred via the U.S. Postal Service. State and County laws regarding the sale and transport of firearms vary.