There are probably more theories and ideas about speargun design than any other piece of kit ranging from the brilliant to the absolutely ridiculous. Here we will try to cut through some of the bull and get to the main points.
Here are a couple of things to remember:
1. Guns always shoot best rigged up as they come off the shelf. Good manufacturers spend a lot of time and effort testing their products so it is logical to assume that they have rigged their guns with the right length rubbers and shafts.
2. If you want more power or distance you need more length. The most important thing for the range and power of your gun is the distance between a slack rubber and a tight one ie the barrel length. Simply adding more rubbers is not going to help.
3. If you miss a lot of shots or are thinking you need a double wrap you are probably shooting from to far away.
4. You cannot save money by buying parts and making your own gun.
The speargun can be broken down into a few main parts, which we will talk about individually, that all need to be perfectly balanced for the gun to shoot straight. When looking at a gun ensure that all the bits seem to fit properly and that the shaft is sitting perfectly straight. It is very common for lesser brands to simply buy discount, mismatched components from other manufacturers and assemble them under a different name.
Think about what your main targets will be and buy a gun accordingly. In NZ the best lengths are from 110-130. If you don't usually target kingfish or larger pelagics it is probably pointless buying anything longer than 110 - If you're only targeting kingies buy a 120-130. Remember that you have to be able to see the end of the gun so if you're usually diving in limited vis don't buy a huge one.
MECHANISM AND HANDLE: The mechanism is the most complex part of the gun. It contains nearly all the moving parts and is usually the first part of the gun to fail. Look for simplicity and big, chunky parts. Will the mech accept a wide variety of shafts or only a certain kind? When you load it does it make a satisfying clunk or is it really tight? Does the shaft fall out when you pull the trigger or do you have to pull or wiggle it? What kind of springs does it have, big solid, stainless ones or silly little steel coil things? How does the line-release work? Don't worry too much about the safety as you'll probably end up ripping it out anyway.
The handle of the gun must fit your hand nicely and feel comfortable. Also look at the material used. Nylon, or even better glass-reinforced nylon, is much stronger than plastic.
BARREL: After the gun length, probably the most important thing is the rigidity of the barrel. A weak barrel leads to flexing and a crooked shot. Strength is determined by the quality of the material used and its wall thickness. Guns should have between 1.8-2mm wall thickness.
The other major thing that increases rigidity is an extruded rail like on Rob Allen guns. This rail stiffens the barrel considerably and is the main reason for having one. There are many imitation "railguns" out there that are just a normal, thin, aluminium tubes with a plastic sleeve over it with a rail. Obviously this adds no stiffness to the barrel, but it actually also reduces the life of the gun as water gets caught between the sleeve and barrel and eats away at the aluminium. The rail does guide the shaft to a very limited extent and makes the length of the spear overhang less important.
It is very important that the barrel is water tight. The best way to do this is to plug each end with a rubber bung. O-rings around the muzzle or handle, or foam is far less effective.
There are three main materials used for barrels, aluminium, carbon and wood:
Aluminium is by far the most common. This is because it is light, very strong and cheap.
Carbon guns are becoming increasingly popular as they offer the same strength with less weight. The ends of carbon tubes however are susceptible to cracking and you should never buy a carbon gun with a male-fitting muzzle or handle without a protective shroud covering the fragile edges. Beware of buying carbon tubes not specifically designed for spearguns as they will often soak up water. Carbon is considerably more expensive than aluminium.
Wooden guns are good once we start wanting to run three or more bands. The added mass of a big wooden barrel reduces the recoil felt when firing 8mm shafts with multiple bands. Lighter wooden guns however aren't so flash and often get significant barrel flex when they are a similar thickness to a normal alloy or carbon barrel. The answer is obviously to go thicker but then you're pushing around a whole lot of extra wood for no reason. Basically unless you are chasing giant tuna you are better of with alloy or carbon.
MUZZLE: There are a few main muzzle designs but with all of them the most important thing is the material. Nylon is best as it is strongest (stronger even than aluminium ones which seem to explode after a couple of years).
One of the main differences is whether it takes screw-in or loop rubbers. Screw-ins are possibly more convenient to change but have more failure points whereas loops are simpler and generally cheaper.
The other major difference is between open and closed muzzles:
Open muzzles rely on the shooting line to hold the shaft in place. They are a cleaner design and have a very clear line of sight when aiming. They also have less shaft slop making them quieter. The rubbers do not however pull in straight line so you should only have an open-muzzle on a railgun. They are also more complicated to reload.
Closed-muzzles hold the shaft in place. They make re-loading very simple and allow in-line rubber pull making them suitable for tube guns. They are available for loop or screw-in rubbers. They do obscure the end of your spear when aiming.
At the end of the day no muzzle is better than the other - except in terms of construction - and it's up to you which you prefer.
SHAFT: The business end of the gun. There are two main materials used, spring steel and stainless. Spring is much stronger and the shafts last longer despite the little rust that they do acculmulate. Stainless is lighter and faster. Generally, in NZ ,spring is far more suitable as we shoot bigger fish than the European ones stainless shafts are designed for. The flopper should be at least 80mm and flop the first 45 degrees easily then jam open. A lot of European spears flop very easily. This makes them less likely to jam open and get caught in the back of a cave which is very useful elsewhere but we don't really fish in caves here in NZ.
The flopper can be fitted to either the top or bottom of the spear and makes little real difference.
Shafts come in various diameters but the most common are 6.5,7 & 7.5mm. As a general rule guns shorter than 120 should have 6.5mm, longer one's should have 7mm and double rubber guns should have 7.5mm.
The rubber power must be balanced by spear weight and you usually can't change one and not the other without compromising accuracy.
Guns should have about 40cm of overhang.
If in doubt just put whatever it came with on.
You should have as few notches in your shaft as possible as this is where, on very, very rare occasions, they snap.
RUBBERS: Rubbers come in so many shapes, colours, sizes and flavours it can be hard to know what the right one to use is. Basically theres two types, natural and synthetic. Natural is far superior to synthetic although it is much more expensive. The colour doesn't really tell you anything about quality.
Then there's screw-ins versus loops, again it's up to you which you prefer although it is often more economical to use loops you can make yourself.
The length and diameter is more important.
Generally you use single,16mm rubbers on tube guns and guns shorter than 110 and 20mm rubbers on longer rail or carbon guns (20mm rubbers are usually too strong for long, aluminium, tube guns)
You can use double 16mm rubbers on guns 120 and up. This will give the gun a little more oomph and make it easier to load.
Generally the rubber length is between 1/4 and a 1/3 the length of the gun barrel. The shorter your rubbers, the longer they will be guiding the shaft and the straighter your shot will be. That is why it is not usually a good idea to put fat rubbers on short guns as they have to be too long for you to be able to load them.
It is also worth noting that your spear will reach a certain speed and then no amount of rubbers will increase that so you're just wasting energy and potentially damaging your gun.
If you do power up your gun you will need to put a heavier shaft on it.
Again what it came with is probably best.
Dyneema is by far the best wishbone material as it is light yet incredibly strong, cheap and finger-friendly.
SHOOTING LINE: There are really only 3 options for shooting line, or 2 good ones; mono, dyneema or some sort of string. Mono is strong, can be crimped, doesn't tangle or twist easily, abrasion resistant, nicks are easily visible and it is cheap. Dyneema is extremely strong, can't be crimped, twists easily, can be abrasion resistant, the core (the important bit) is hidden and it is relatively expensive. String is crap.
Here at Wild Blue we are totally and unashamedly biased towards Rob Allen spearguns. This is simply because in our, and our 1000's of customers', experience they outperform and outlast any others. If you'd like to read more about Rob Allen spearguns and why they're so good click here