The entire top part angles and slides out and when it does that you can put the bullet into the rear of the barrel (No collision but still opaque, i'll fix this later)
Make sure the bullet stays in place, otherwise the killer wont hit the bullet and nothing is gonna fire
If you've loaded the bullet sucessfully, pull the trigger to fire (duh, what else is gonna fire it?)
I found a solution (for win10), go into settings, ease of access, then go to the interaction tab, click mouse and disable "Turn on Mouse Keys to use the numeric keypad to move the mouse pointer", No need to thank me, works for me
Runtime: 6.2
Everything else:
discontinued(!) Lenovo v510z with airflow so small its practically passively cooled
CPU: Intel i5 6400-T (boost up to 2.71ghz)
GPU: none (Intel HD Graphics 530)
RAM: 8GB SODIMM single-stick RAM @2133mhz
OS: Win10 Home edition