Then grab the fish by the eye sockets ,with gloves,and remove the spear. There are two basic types of guns .The band powered and the pneumatic. Each has it advantages. The larger band guns are the most powerful but take longer to reload. Pneumatic guns have good power but can be reloaded faster.
Size is a matter of application.
Usually longer guns have more power ,longer range and are more accurate. Smaller guns can be more conveivent for smaller fish. As a rule the larger the fish your after the bigger the gun needed to penatrate. Pole spears are an inexpensive alternative They have good power at close range and can be reloaded the fastest. There are many types of spear tips on the market.