Yes, there are different length nipples, with different thread sizes. You have to remove the nipple to expose the threads, so that you can measure them with metric thread gauges( see your local hardware, or auto supply stores) to get the size correct for your gun.
Use a nipple wrench to remove the nipple, and put it back on. If its "stuck" soak it with a penetrating oil, like Break Free, Liquid Wrench, or Kroil. Even Break cleaner works, as does kerosene. Don't hesitate to pour some some of this penetrating oil down the barrel, while you put a tooth pick into the nipple to seal that hole, and stand the barrel in the corner some place for a week! Let the oil do its job. When all else fails, applying heat to the bolster or drum( where the nipple is screwed into the barrel, to expand that metal while turning the screw out, will get the job done. If all else fails, you may have to drill the nipple out.
It happens. Sometimes someone has bought the wrong sized nipple, and cross-threaded it into the bolster! :nono: :shake: :shocked2: Don 't hesitate to take the barrel to a qualified gunsmith, or machinist with a good drill press to do this work. :hatsoff: