The pro AR-15 argument comes down to “preventing state tyranny.” In other words, “I need an AR-15 because one day I might have to mow down a bunch of U.S. soldiers. Don’t get me wrong, I support the troops. I just want to be ready to murder as many as possible if necessary.”
Their argument is wrong, but it's more subtle than that. They want to be able to override law/courts by pulling a Bundy - use threat of massive loss on both sides to prevent enforcement against them.