In ULTRAKILL, there is an extremely useful weapon called the “saw blade launcher.” This weapon is an alternate version of the nail gun; it’s alternate fire can shoot a metal pole, and primary fire shoots saw blades, which will spin around the pole rapidly, dealing massive amounts of damage to any unfortunate soul who happens to be in the way of the saws. Unlocking this weapon is fairly easy as well, and this guide will explain how to do just that.
First, travel to 4-4, and walk on those blue bounce pads near the entrance to be launched upwards. Once at the top of the hole, there will be a hallway to the left, a locked door in the middle, and a slightly cracked open door to the right. Walk up to the cracked open door, and there will be a skull on a pedestal.
Use the whiplash arm (the one obtained from fighting V2 the second time) to obtain the blue skull. Once done, aim the electric rail gun here as shown.
After firing, the sound of doors can be heard opening; return back to the locked door in the middle and it will be open. Proceed through, and the saw blade launcher can be found – along with another surprise (one that is best left unspoiled).
That is all it takes to unlock the special saw blade launcher in ULTRAKILL. While the process to unlock it is simple, it does not mean the weapon is weak – in fact, it is quite the contrary; the saw blade launcher is an incredibly powerful tool that can even be used to help cut down the mighty Minos Prime himself (a guide on how to beat him can be found here).