How To Measure Speargun Rubber Size

Some of the most commonly asked questions we get are: "What speargun rubber best suits my speargun length?", quickly followed up with "What thickness speargun rubbers should I use?"  The answers to both these questions depends on four main factors: The mass of the speargun. The individual skill level of the spearo/shooter. The thickness diameter of the spear shaft. And finally, the actual mechanism being used. In this section we'll look more closely at just how each of these factors can affect the handling of the speargun and how it directly relates to the selection of speargun rubbers.

Mass of the speargun

The gun must have the appropriate mass to handle the recoil generated when the speargun is discharged. A common problem with in-experienced divers is to load up the gun in the belief that they can shoot further and secure more fish. This belief is false.

If the speargun mass is too light for the selected rubber then the recoil produced on discharge will sharply raise the muzzle as the spear is leaving the mechanism there by kicking the rear of the spear up and sending it out in a downward trajectory. This results in a shot that is low and inaccurate and the inexperienced diver falsely thinking that he needs even more power! Therefore always consider the mass of your speargun when selecting speargun rubber lengths and thickness.

Spearo skill level

As we all know everyone is different. Knowing this, then it naturally follows that all spearos will also be different. They'll have different levels of experience, different skill sets and different capabilities. Most importantly, it means that as shooters they'll also vary in their ability to handle recoil.

The ability to handle recoil is directly affected by both technique and individual strength. This means that a speargun that's been personalised and custom setup to be one spearos dream could easily be another spearos worst nightmare. So always remember to carefully consider your skill level when selecting speargun rubber lengths and thickness.

Diameter of spearshaft

The diameter of the spearshaft is the determining factor here because a steel spear doesn't behave itself like a spinning bullet leaving a rifle. A spearshaft when fired will shoot forward and wander through the water once released. This is due to the elastic nature of the steel that's used in the construction of the spearshaft itself. By comparison, a thinner spear when fired will quickly lose accuracy control sooner than when compared to the firing of a thicker spear.

When the trigger is squeezed the rubber power bands come into action and apply an enormous accelerative force to the spear. This sudden acceleration combined with the spears natural inertia and elasticity causes the spear to hump up and jump as the back of the spear tries to catch up with the front of the spear. As the spear flies this humping effect creates an oscillation or wobble in the spear and it snakes through the trajectory. You can observe the results of this oscillation on a well used rail gun, there are scuff marks at regular intervals along the rail where the spear bounced through. For this reason it's highly important to consider spear thickness when selecting speargun rubber lengths and thickness.

Type of mechanism

The type of mechanism in use is highly important and by far the most important factor to consider when selecting rubber size and thickness. If the wrong size or thickness of rubber is used on the wrong mechanism it could quickly become a huge safety issue that could have devastating consequences. It's highly important to know that all speargun mechanisms have a maximum load rating that has been directly prescribed from the manufacturer and the engineers who created the mechanism. These max load ratings have been developed under strict testing conditions and regimes which need to be strictly adhered to.

Therefore NEVER overloaded a speargun mechanism to avoid misfires and total mechanism failure. ALWAYS familiarise yourself with the maximum load ratings of each speargun that you intend to use and remember that max load ratings do vary from brand to brand and even model to model.

All spearguns that accept a European styled shaft, which is a shaft with a rounded cut receiver end, should never be loaded with more than two 16mm power bands. The reason is that this type of mechanism works on a slip principle, i.e. the spear slips of the sear when the trigger is depressed. Do not listen to any one or any advertisement that tells you different. Therefore always consider the type of mechanism and its max load rating when selecting speargun rubber lengths and thickness.

Conclusion

The length and diameter of the speargun rubbers will certainly affect the overall power of the speargun as well as its safety and longevity. It's important to remember that incorrect rubber selection can have significant flow on affects on both the range and the accuracy of the speargun. Therefore always be sure to carefully consider the mass of the speargun when selecting new speargun rubber lengths and thickness. Be honest with yourself and take into account your individual skill level and experience. Take into account spear thickness and how it will affect accuracy, range and penetration. Finally, NEVER overload a speargun mechanism to avoid misfires and total mechanism failure. By following these steps you're sure to have great success when selecting the appropriate rubber thickness and rubber lengths for your guns.

Example

The following is a recommendation only and is based on creating a reliable, easy to load, accurate, hard hitting speargun that will work best for an all rounder power ratio and an intermediate spearo with intermediate strength. Shorter lengths will give additional power however, always be careful not to over load the max power rating and strive to not overpower the speargun as this will decrease accuracy and make re-loading significantly harder. Remember that when you shorten a rubber this will mean that the rubber has to be put under more tension to stretch it along the length of the barrel and this will also increase the power of the shot. This can result in additional penetration power for your spearhead for larger fish species.

As an example, a 90cm speargun using a 16mm rubber that measures 50cm with a 7mm spear shaft will provide the spearo with a standard, medium strength power which is ideal for reef and bay spearfishing. If hunting larger fish and wanting more power, consider shortening the length of the rubber down to 46cm depending on your own level of experience and strength. If you're not highly experienced or particularly strong then consider only adjusting the rubber size down to say perhaps 48cm. What ever you may decide, based on your own individual limitations and the limitations of your equipment, you now have a guide to work to.

Remember that when you shorten a rubber this will mean that the rubber has to be put under more tension to stretch it along the length of the barrel and this will also increase the power of the shot. This can result in additional penetration power for your spearhead for larger fish species.

The following is a recommendation only and are the lengths that work best for an all rounder power ratio. Shorter lengths will give additional power however, always be careful not to over load the max power rating and to strive to not overpower the speargun as this will decrease accuracy and make re-loading harder.

                               Rob Allen Rubber Gun Sizing Chart: