Just converted my 1.4 rail gun to a gun that is hopefully capable of landing a bigger fish like Tuna or Doggie. I replaced the 7 mm spear with an 8 mm shaft, a Daryll Wong slip tip, locally made by the way, and a breakaway rig. Pool testing shows that the spear is remarkably accurate but does drop slightly when shooting from distances greater than 4 metres. Currently using a 20 mm rubber 870 mm length(total) Was thinking of going 2 x 20 mm but that may be too strong and cause the spear to damage the trigger release mech. which would not end well. Would 2 x 16 mm rubbers provide more power? 2 X 18 mm? Can only go to max. of two bands. Planning on using a 2 metre bungy onto a surf lifesaving foam filled float followed by a 5 metre bungy and a 35 litre air filled float, hope that this will provide enough resistance.