Easiest way to put it is this way.

Rifle - Long distance shots. Semiautomatic rifles like AR-15 types put out a lot of bullets with precision and are a good way to protect a perimeter.

Pistol - Medium distance. Kill or wound in situations where you can see the white of their eyes.

Shotgun - Short distance (indoors) and highly versatile. You can even load it with "less lethal" ammo.

Knife/baton - Hand to hand combat.

I am not a hardcore gun aficionado so please, I encourage further feedback.