Thanks for playing our game and giving feedback!
Shotgun's aim is not as precise as pistols but if you aim and shoot in close range you should kill a monster in one shot even without attaching upgrade part. Idk why that happened, I assume that you aim before shooting because weapons shoot their forward direction and not yours, without aiming shotgun mostly shoots your left.
And for pipe, it can kill monsters but not very powerful and you should use it in some spesific cases (like when you run out of ammo, when your weapons stuck and there's nowhere to run or maybe after tearing both of a monster's arms so it can't attack you).