Double weapons generally fire a pair of shots simultaneously, one forward, one in another direction. In most Gradius titles, a given source cannot fire another volley until both shots have disappeared, taking away dependability and power. Most double types are utility weapons that should be switched away from when their purpose is served, though some exceptions exist.
Double: The extra shot goes 45 degrees upward. It appears in almost every Gradius game on the default type/ship. Upward firepower that can ease the difficulty of certain stages.
Tail Gun: The extra shot goes 180 degrees backwards. Some stages have many enemies that attack from behind, making this ideal for them.
Vertical: The extra shot goes 90 degrees straight up. For reaching entrenched targets directly above. ( Gradius III and V)
Free Way: The extra shot goes in the last direction you moved in, being able to fire in one of 8 directions. Awkward, but can replace your main weapon and is extremely versatile. ( Gradius III AC & Gradius V)
Spread Gun: Fires two shots in a spread pattern, and can be upgraded to fire three. A sort of pseudo Ripple Laser.
Back Double: The other shot goes 45 degrees backward. (Gradius III SNES)
Twin Cannon: Kind of like the Twin Laser in other games, except the projectiles are normal shots.(Gradius: The Interstellar Assault)
Round Laser: Upon firing, a growing circle laser surrounds the ship. Still fires a regular bullet.
Auto Aiming: The extra shot targets enemies from front to back, with the shots disappearing at a certain distance. Fires a regular bullet.
V-Shot: A shot goes up and another goes down. (Gradius ReBirth)