You don’t have to argue about the meaning of “weapon.” The law the FAA cites defines “dangerous weapon” exactly as quoted in the article. Intent to use as a weapon isn’t a requirement. The fact that other devices might arguably meet the definition isn’t going to matter to the arresting officer or the prosecutor. Ultimately it will be up to a judge and jury to decide if one of these flamethrowers meets the definition, but unless you have a real, provable legitimate purpose for it, I wouldn’t want to roll those dice. A legitimate user can and should go through the hoops to get approval, not just for the flamethrower but for the non-hobby drone use.