There are some things that just aren't worth messing with. The BATF is looking for any excuse to nail people with 10 years in prison and/or a $250,000 fine. Owning the semi-auto and the book together are fully legal, but it shows interest in doing something illegal, which while not illegal in itself, may attract painful attention.
If he wants a bullet hose, he should find out how to do it legally. Depending on what state he's in, he should be able to buy one (expensive, but entirely possible).
Better to generate circumstantial evidence of a strong desire to obey the law than to the contrary.
Remember: the job of the police is not to arrest those breaking the law, but to arrest those that the police _think_ are breaking the law. It's up to the courts to determine precise legalities.