Some airlines ask you to show them the sports equiptement (golf clubs, etc) if you use luggage for that sport, because they charge you more if you stick anything else inside (united airlines off the top of my head). Many airlines list what is allowed in the bag and if you have anything else they charge you as oversized luggage which is considerably more than you would have been charged (if at all).
However, they all define acceptable checked luggage as being no bigger than 158cm linear (h+w+d). If you travel with a short speargun (75cm) and travel light (stick everything acceptable in your carryon luggage) you can get a sportube series 1 or 2 (and shorten it) to use as your only checked (free) luggage. A series 1 can be set to 127.2cm and thus accept a 120cm spear or maybe even a 130cm spear in diagonal. A series 2 can be shortened to 114.2cm and be in the acceptable size with a 115cm spear in diagonal.
If you stick the spear in diagonal you can make it even shorter, use this formula to calculate the minimum length/height needed to fit your spear and then use that to see if it fits in the authorized size : l = sqrt(s²-(w²+d²)), where l is the max possible height/lenght of the luggage, s is the spear length (longest item), w is the width and d is the depth. I hope this makes sense and helps someone :).
BTW when you shorten a sportube you loose the straps inside, not a big loss. The series 3 is too large in theorie, it can barely accept a 115cm spear in diagonal at it's smallest size and it is still too big (163.8cm), if you have a 110cm spear you can shorten it to 101.7 and you would be at 158.5cm which should slide if they measure.