Niko's Roller Gun Page

This is my Roller Gun II (read about RG-I here) in the unloaded state. The rubbers are under the barrel when the gun is not loaded. When the gun is loaded, the rubbers pull over the rollers at the muzzle, back towards the handle in the usual way. Why? There are two main reasons:

1. On a normal gun, about 1/3 of the barrel length is wasted, to accomodate the slack rubber. What the rollers effectively achieve is to fold the slack part of the rubbers around to below the barrel. Ignoring roller friction etc, this gun now has the energy output of a gun with a barrel 1.3 times as long.
2. A long aluminium barrel on a gun with strong rubbers tends to bend upwards when loaded, because the rubbers pull above the barrel and not below. When you shoot this causes energy loss and may cause inaccuracy. What the rollers achieve is to equalize the force above and below the barrel, so that it cannot bend anymore.

Caution! If you try to build a roller gun, remember the following: If the force of your rubbers is 50kg say, the force on the axle will be double that, 100kg, because the rubbers below the barrel pull just as hard! Those rollers badly want to accelerate towards your face! You have to construct a strong muzzle.

A possible problem with the roller gun can occur because one can be tempted to construct a short powerful gun. I found this out the hard way with my first model. (I lost a 12kg yellowtail when the spear failed to penetrate the spine.) I built a short gun with 7mm x 1.3m spear. With the rollers, I calculated this gun would be as powerful as my favourite normal gun with 7mm x 1.7m spear. These two guns probably had close to the same muzzle energy, but I had neglected to consider the effect of spear mass. The shorter spear is lighter, travels faster and loses its energy over a much shorter distance than the heavier spear. On the second model I used an 8mm x 1.5m spear which is a bit heavier than my favourite 1.7m spear.

I used a Rob Allen aluminium barrel with full length rail. Louis Hattingh of Rabitech helped me fit the handle by turning an adaptor on his lathe.

I usually load my guns by pulling on the rubber with my left hand while pushing the handle with my right until the handle is against my chest. Then I reach the rubbers with my right hand as well and pull the rubbers all the way with both hands. With the roller gun I cannot reach with my right hand, because my left cannot pull far enough. I had to mount a hook on the barrel to make a two stage loading process possible. You can see the fibreglass cuff in which the hook is mounted in the above picture, near my left hand. I push the handle into my groin and pull the rubbers with both hands until I can hook them. Then I put the handle against my chest and pull the rubbers all the way back with both hands.

Below is detial of the cuff. The coloured lines to the left hold the rubbers. The webbing to the right is fixed to the handle.

The cuff can slide along the barrel, so that it transfers the rubber tension all the way back to the handle, to prevent the barrel bending. (Although the cuff can slide, it moves very little during loading and firing - it is just the bit of stretch of the webbing that allows it to move.)




The muzzle with rollers seen from the front. The rubbers are in the unloaded position below the barrel. I fittted a webbing guard to catch the rubbers so that they don't fall off the gun when firing. (In later tests I found the guard is not necessary.) The rollers I turned from acetal (using a hand drill and a file). Acetal is a durable, hard, almost metal-like polymer with a very low friction coefficient. The roller axle is a length of polished 7mm stainless spring steel. (No prizes for guessing the source.)


The top of the muzzle, unloaded. The nozzle is longer than is necessary to hold the spear in place, it was designed to hold the webbing guard. However, I found that it is in itself long enough to stop the rubber jumping over when firing, so I have removed the guard.


This is the wrong end to be standing when the gun is loaded. The rubbers are on the hook, not in the spear :-)

You may have noticed that the rubbers don't stretch along the entire length of the bottom of the barrel. Can't you get more energy by using the whole barrel? Theoretically yes, but this is impractical. I kept the rubbers short so that the percentage elongation of the rubbers would give me the required maximum tension for the rubbers. If I would use rubbers with a slack length of the whole barrel, I would only get a 200% elongation, which would not give enough maximum tension.

If, however, the slack rubbers were fixed to the base of the barrel (at the handle) and come to about 2/3 along the bottom of the barrel, I would get the required 300% elongation. In this case the rubbers at the rollers would already have some tension. This is illustated in the diagram, where you get the yellow area being equivalent to the muzzle energy. The problem is the rubber tension at the rollers makes the gun really difficult to load in this case. I have settled for just a slight pre-tension at the rollers, in order to keep the rubbers from flapping around too much in the unloaded state.

How does the gun shoot? Is this design worth the extra cost? Won't it be simpler just to fit double rubbers? Do the rollers affect accuracy? I don't know yet. I would have loved to publish some fish photos here shot with this gun, but we have had a disastrous 2000 summer season. The weather has been wrong for spearfishing for about 4 months now. (Usually we get warm clean water with lots of yellowtail in summer, but an unusual weather pattern has caused the water to turn cold and dirty. This is probably the same weather pattern that caused the floods in Mozambique.)

I can make a comment about double rubbers. The muzzle energy of a gun is the force of the rubbers integrated along the effective length of the barrel. The effective length is the barrel lengh minus the part where the rubbers are slack. If we assume the rubber force has a triangular graph, with maximum force where the rubbers are hooked and zero force where the rubber becomes slack, the energy is the area under this graph. This is equal to 1/2 times the effective barrel length times the maximum rubber force. With double rubbers you increase the maximum rubber force. With my roller design, the maximum force stays the same, but the effective barrel lengh increases. The problem in the case of 2 rubbers is that this increased force tends to bend the barrel. As explained above, the roller gun does not have this problem.

I have tested the gun in my small pool. I had to fire the gun into a 4.5m PVC tube, blocked with a length of wooden pole wedged in the end. The spear travelled through at least 4m of water. The tip penetrated between the PVC and the wood, splitting the pipe and wedging itself so firmly that I could not pull it out by hand. I'll publish some more test results here as soon as I have had the opportunity to use the gun.

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