Roller speargun

On a normal band gun the rubber stops pulling when it reaches its relaxed length, well before the shaft leaves the muzzle: the last part of the barrel is wasted. A roller gun runs the band round pulleys at the muzzle and anchors it under the barrel, so the rubber is still stretched, and still pulling, as the shaft exits. The result is a longer power stroke and a faster shaft from the same barrel, with less of the kick a stiff double-banded gun gives.

The practical effect: an 85 cm roller shoots like a 100 cm conventional gun, which lets a diver keep a short, manoeuvrable gun on rock and still reach a fish that holds off.

A roller gun is a second gun, not a first. The extra range invites long shots, the loading is a two-stage job that takes practice, and the mechanism has more to go wrong. A diver who has learnt to get close on a plain band gun gains from a roller; a beginner gains a way to miss at longer range.

What matters:

• Barrel length: choose as for a band gun but one step shorter, since the roller adds the reach.
• Pulleys: smooth, corrosion-proof, easy to strip and clean. Sand in a pulley is the usual failure.
• Pre-tension: an adjustable anchor under the barrel lets you set how much stretch the band keeps at rest; more pre-tension, more power, more effort.
• Loading: some rollers load in one pull, most in two; try loading one in the shop with a pad on, because the second stage is where divers give up.
• Shaft: rollers push a shaft hard; a 7 mm shaft with a good tip is the usual pairing, and a bent shaft is the usual symptom of a shaft that was too thin.

Inverted rollers, where the band runs under the barrel and doubles back, are a further step in power and complexity; they belong to blue-water hunting.

One step shorter than the band gun you would choose for the same water: 75 to 85 cm for Moroccan rock, 90 to 100 cm for clearer water.

The pulleys and the anchor take the salt and the sand. Rinse thoroughly, spin the pulleys under fresh water, and strip them if they feel gritty. Bands on a roller are always under some tension and age faster; inspect them every session and change them each season.