Yes I did slightly simplify. There is an exception for buying rifles and shotguns from a gun store, but that still requires passing a background check and everything. Also the rifle or shotgun must be legal in the state you reside. And it is illegal to privately sell a gun to a non-resident. If the buyer is from another state, you must go through an FFL.

The point I was trying to make is that people cannot take advantage of less restrictive gun laws in neighboring states. Almost every mass shooter either got their guns legally or stole them from a close relative (often by murdering that relative). There's no glaring legal loophole that these people are using.