I'm extremely skeptical of the reasoning you lay out, and I'm extremely skeptical of the good faith of an association that brands itself like the FPC does (pictures of happy people holding assault rifles, "WE'RE FIGHTING FOR YOUR RIGHTS") when debating a policy that results in any limitation whatsoever in access to firearms.
Saying "okay, but the definition of what is an 'assault rifle' is murky" isn't a good faith rebuttal, because the endgame of the people making that rebuttal isn't "therefore we should find a more clear-cut way to define what firearms are too dangerous for people to have access to and pass efficient legislation to limit that access", it's "therefore we should let people have whatever weapons they want".
Saying "okay, but the definition of what is an 'assault rifle' is murky" isn't a good faith rebuttal, because the endgame of the people making that rebuttal isn't "therefore we should find a more clear-cut way to define what firearms are too dangerous for people to have access to and pass efficient legislation to limit that access", it's "therefore we should let people have whatever weapons they want".