First level
Keys gives you the pistol
As soon as you have control of MC again run as fast as you can to the grunts and make turns as fast as possible
since you went so fast that you skipped the checkpoint, you never get your pistol
there will not be grunts
go through door and grab assault rifle
go into chayos and grab a dead persons gun (any gun will do for now, you can always swap it later)
now go back to keys, and then go back to where your supposed to go
you will pass the checkpoint where you get the pistol, but since you already have 2 weapons, you get the pistol added to your inventory also and you now have 3 weapons that you can cycle through
the grunts will be there now, so kill them and go through the door as normal and go through the game as normal