spear
鈒 centers on a spear, a pointed weapon used for thrusting or throwing.
鈒 combines 金 (metal) with 及, which likely contributes the sound. The character is used for a metal spear, but the exact historical development is uncertain.
Metal 金 combined with 及, which looks like a hand reaching for something, suggests a metal weapon that extends your reach: a spear.
For ソウ, imagine a soldier named Sou thrusting a metal spear: Sou -> ソウ, and the spear is made of 金.