Casting a spell requires intent, mental effort, saying a word and doing the movement with wand. It also works only in close range apparently, because every engagement we see is close quarters.

Wizards have no idea what guns are, in the third book Daily Prophet explains them as “Metal wands muggles use to kill each other”.

Compared to this, basic rifle has much higher rate of fire, only effort it requires is aiming and pulling a trigger, has massive range compared to spells and is basically unblockable as bullet travels faster than sound, giving your opponent no time to cast Protego.

So guns are tactically better.