Aiming a speargun differs from person to person. Many old divers have had a problem trying to get used to railguns. I put this down to the way they were used to aiming the old non-rail type gun. A common method is to look at the tip of the spear, put this on the spot you want to aim at and then lift the handle up. As the tip disappears from sight, pull the trigger.
With the railgun having a shorter spear, if you try the same thing, the tip will disappear sooner behind the muzzle, as the spear is shorter. Therefore the handle would not have been raised enough and you now shoot over the target. Some compensate by fitting a longer spear but this affects the speed of the spear. The original railgun was designed to take even shorter spears than are now fitted. Customers did not like the spears that short. Too radical a change from what they were used to, but they do shoot fine. If an older diver used a completely instinctive method of shooting with no sighting at all there is no problem to convert between guns. Shooting at reef fish usually requires either instinctive or “point and tilt” aiming because there is often less time to prepare for the shot.
I have mainly hunted gamefish and before the advent of railguns I used to aim along the side of the gun, sometimes even turning the gun onto its side. (You can't do this with a railgun as the spear tips out the rail.) By aiming while looking along the side of the barrel you control the vertical axis better than the lateral. With game fish being long and thin, the vertical plane is obviously much more important than the lateral. When old guys had problems aiming the rail gun, I tried to get them to use this method but most would not change.
I needed to get into a pool and just shoot at a target. I needed to see just where the spear goes and what aiming method is best for pool and the ocean. In a pool with the pumps off, no disturbance you can tell exactly where the spear is going, as there are no other influences such as current, swell, moving target etc. To be able to do ongoing pool tests I built a large pool at home with sufficient length to shoot long guns. I soon picked up a few quirks. The main finding was just how much recoil affects the shot and, just how easy it is to adjust the flight of the spear by tweaking the barb. Another major factor was just how important a straight spear is. A very slight bent in the tip will set the spear off target a long way. This very small bend is almost impossible to see but can be detected when rolling the spear on two edges with the line disconnected and the barb taped up.
Rubber power was a big influence. I found that if the gun was powered up, the recoil caused the gun to shoot left or right, not so much up or down as one would think as with a handgun. How much deflection and which way was relative to how tight I held the gun, how stiff my arm was, which hand I held the gun in and, the power of the rubbers. I am left-handed and the recoil deflection for me is to the right. If I hold the gun handle with my right hand, it deflects to the left. I put this down to the way the handle kicks back into the thumb and then rotates the wrist.
With a 1,3m gun with double 16mm rubbers, set up with a 7,5mm spear, rubbers a little shorter than normal, the left or right deflection is as much as 100mm at max range. My definition of max range is the range a spear will travel and just come out the other side of a good fish. I set up the target such that the spear tip will only go through it and protrude out the other side by about 300mm once stopped by the shooting line. (Single wrap) This 100mm deflection is with a normal grasp of the handle. It improves to about 50mm with a tight grip and stiff-arm but is 150 to 200mm with a slack grip. My personal game fish guns are 1,3m with normal double 16mm rubbers and a 7mm spear. With this set up the lateral deflection at max range is only about 30mm with a normal grip. This is nothing at max range on a good size fish.
Have you ever missed a fish at close range? This happens from time to time, you think it is an easy shot but then you miss totally. I put this down to the recoil. The shot was a dead cert so you just relax and pull the trigger. In this “relaxed” state the gun recoils so much more and… you miss.
To get the vertical accuracy right, I always sight the same way and adjust the barb accordingly. The trailing edge of the barb can be tweaked to raise or lower the spears flight. We used to bend the trailing edge of the barb out a little to help toggle on a fish. This was fine with spears made from 1840MPa steel that was more flexible. With the new 2100Mpa steel we now use, when shot, the spear would hit high with that barb setup like this so we stopped doing it. In the pool I found that a half mm (0,5mm) outward bend of the trailing edge of the barb would lift the spear on my 1,3m gun at max range by as much as 50mm to 70mm.
The way I now aim is to look over the top of my gun. You can't use the top edge of the handle to line up with as the rubbers sit higher than this. For me to sight properly, (using sighting points like a rifle) I look over the top of the gun and line up the rear rubbers with the muzzle (when loaded). When loaded the rubber creates a “V” like image between them. This is the “V” between the two diameters when looked at end on. The normal closed muzzle I line up such that the middle muzzle hole, the one the spear goes through, sits on this V as you would when sighting a rifle. If you try this and it still shoots high, lower the hole into the “V” more.
I practice sighting like this often so it feels natural when I go into the ocean. If you don't, it will feel strange. In the pool you will need to take many shots for it to feel right, it does get tiring loading and reloading 20 to 25 times in a row, but it is necessary. Make sure you are wearing a good loading pad. I also always set my shooting line the same way and always set the line to the same side of the spear. Once loaded I also pull the shooting line out from under the rubbers as this helps prevent muzzle wrap. It is a good idea to get into a routine like this. I always try to pool test every new rubber and or spear I fit to my gun before going on a trip. This also gets my eye in, very important before a trip. It costs a lot to go on a trip in terms of time and money; you don't need to spend the first few days getting your eye in or second-guessing your aiming technique. You might miss that one good fish – it’s just not worth it.
I now use an open muzzle that makes it even easier for me to aim as I can see past the muzzle, right to the end of the spear. With open muzzles I sight with the edges of the barb that I see when looking down the spear. I just “rest” these on the "V" of the rubbers at the back. The open muzzle is a little more difficult to load the spear line but once used a few times it becomes easier. When changing to an open muzzle it is a good idea to first pool test so you can get familiar with the spear loading and line wrap. It is not a good idea to try this in the ocean while there are fish about.
Point of interest, I have had several novices in my pool who had never shot a spear gun before. I load, hand it to then, then only tell them to keep their arm out straight, hold the handle tight and shoot, all shoot almost spot on. This shows that the way "old divers" aim is the problem, not necessarily the gun.