Guns have an effective range and a total range. Just like a gunpowder type of gun it depends on the Caliber(size & length of shaft), the power propelling it (bands/pneumatic pressure), water conditions (currents, viz), enviroment (wrecks, ledges, rigs), and line/free shaft.
In general big gun, ie 70 inches, long range. Short gun 30 inches, short range.
Just my experience, but I've seen a 70 inch free shaft gun kill a grouper from 35 feet away and I've seen a 30 inch pneumatic bounce a shaft off a barracuda from 5 feet away. And just like powder guns, a 22 caliber pistol will kill just as good if not better than a 44 magnum. When it comes to spearing, accuracy is always better than power.