The nut does NOT have a pronounced taper to accept the ferrule. Just a smooth ridge to push against the ferrule, as the "sealing" is accomplished where the ferrule seats in the sediment bowl/carb. If you ever have a rough seat in the sediment bowl/carb, take a piece of old tubing with a ferrule attached, dab valve grinding on the ferrule, stick it in and turn it back & forth to smooth the seat, like: 1001 1002 1003 1004 1005 106----------You know what I mean!
Someone stated awhile back that on the attached type fitting, he stuck a 1/4" drill bit shank in the ferrule, snapped it off & installed it in the usual manner. Vaseline's good for more than one thing!! puttin' some on the ferrule helps it "squuuueze" to the tube easier. When you're first puttin' a new ferrule, tighten until it "feels" tight, then use a tubing wrench at 3:00 position, push down with straight arm, using body weight, and you'll feel it "give as the ferrule "squeezes" onto the tube, gotta' look like a wedding ring on a fat man's finger!
I type slooooow, I hope I don't have to ever type all this again. (just use search!) At least I've learned how to spell FERRULE correctly!
Some have resorted to soldering ferrules to the line. This is sometimes need on an original line that has a "lap" seam from original manufacturing methods, instead of a "butt" seam that was used later. Bill W.