All secret Beam Gun locations

When you get to the area with the double shotgun, there's a jump pad to your right. Use it to get to to an area with a super health, and in said area, behind the building is the first beam gun.

After you leave the long corridor, there's this large area with crates to your right. Go to the crates and you will notice that the building has a passage that leads to the beam gun. Beware of the Clowns that spawn after you pick it up!

After going through a bridge, you will come across a field with a Giant Spider Mech to your right. Move to your left, jump and immediately go left again mid-jump when you reach the shield wall blocking your path (or alternatively, you can use crates) and you will notice a tree. Behind the tree is the final beam gun.