Everyone knows that yandere don't use guns for fun, but unless they're tired of playing around. So, I'm gonna list which gun they would use for.

-ShotGuns-
Full Burst guns that give someone a big hole. A shotgun is a short ranged gun, so they have to get close to do damage.

-Rifles-
Guns that fire more bullets and has a clip to be exact. Rifles would be used to kill more than one, a big crowd of people. Some rifles can high fire rate, to kill someone fast, or more bullet capacity, to use on more people, or if the enemy has their firearms. 

-Sniper Rifles-
Useful for silent kills, or to kill people far away. Some sniper rifles have silencers, muting their shot, but shortening their range. 

-SMGs-
SubMachine guns to be exact. An SMG is a small rifles, small and fast. It's very useful if a Yandere needs a weapon and has snapped. An SMG can have high fire rate, and even high bullet capacity in it's clip. If your yandere has a suspicious looking figure in her/his backpack, then hide the second they snap. I also recall Yuno Gasai using an MP5 once.

-Pistol-
Of course, the small and reliable, pistol. Like the SMG, small and fit in a backpack, but it's small and is said to be easier to hold and have accuracy, some pistols are easy to dual wield, but some yanderes can have low experience with firearms, and can end up missing, learning how to dual wield pistols can be hard.  

-Revolvers-
Like pistols, small and strong. But Most revolvers are very strong, and kill someone if even shot in the stomach, they're also more rare to be used by yanderes. Yuno Gasai, used a Magnum 43, as i recall. 

I would put Machine Guns but i doubt even a gun yandere would use it, they're supposed to cost a fortune, and missile launchers are NOT easy to fine.