3.Anything worth shooting is worth shooting twice. Ammo is cheap. Life is expensive.
Double-tap if possible.
4.Only hits count. Close doesn't count. The only thing worse than a miss is a slow miss.
One needs only to use caliber and ammunition of such that follow up shot is not required. The 9mm is far to small and inherently underpowered, I do not own one. A plus P .38 in full wadcutter is devastating and very effective. Samuel Colt had the perfect start in 1872 in .45 colt.