Rollerguns have quickly become extremely popular among spearfishermen, and for good reason. In this video Adreno's Wayne Judge talks about the benefits of rollerguns, also touching on inverted rollerguns.
Rollerguns are perfect for providing extra power in gun that is substantially shorter than other guns. This makes it great for hunting bigger fish in cloudy water and can even be used in blue water environments.
The reason that it has the ability to perform in these ways is the nature of the shaft propulsion. The rollers on the speargun allow the rubber to continue to provide tension right until the spear is released from the barrel tip, as they rubbers are pulled over the rollers. The traditional speargun design provides tension for two thirds of the shaft before becoming slack as the rubbers are pulled towards the tip of the shaft, and so the barrel length is not exploited to its full potential.
Another great characteristic of the roller gun is its natural ability to counter the recoil of the gun. Most recoil is resultant from both the powerbands and the shaft propelling towards one end of the shaft with momentum that needs to be countered by the gun and the shooter resulting in the gun recoiling in the opposite direction. However, on a roller gun there is movement in the opposite direction as the bands move over the roller back towards the handle of the gun. This dissipates some of the momentum required to occur in the backward direction in a smoother motion reducing the recoil of the gun and allowing for a smoother shot for the same power in a lighter gun.
The inverted roller gun is controlled pulleys on the underside of the gun which allow for easier loading of the gun. The rubbers are pulled from the underside of the gun and one from the top, making it a more complex gun to load and manage. However, this is also an effective form of roller gun and great for hunting tuna in blue water conditions.