NOTE: Easiest to produce with longer barreled rifles (Navid, Zafir, MX, ect) for reasons explained within. this can be reproduced on any vertical surface so long as it isn't too thick.
Make sure you are in first person
1) find a Command tower (or any other command building, the ones with the giant numbers on the side work best).
2) up the stairs into the first floor.
3) lower weapon
4) walk up to any solid wall (no windows or hatches is best) so that you are parallel with the normal of said wall.
5) try to walk through the wall to make sure you are as close as you can get
6) turn 90 to the left so that your right shoulder is against the wall
7) strafe right into the wall to make sure you are still flush to the wall
8) turn on free look (hold / double tap left alt)
9) toggle on lean right (double tap e)
10) move mouse to the right
11) hold right mouse button (zoom)
At this point, the first person camera should clip through the wall, allowing you a free view of what is happening outside without exposing any part of your body to harm. From the outside, depending on the surface you clipped through, there may be a sliver of your helmet sticking through the wall, but in most cases you are totally safe from any kind of return fire.
In order to shoot through walls, follow these steps.
1) walk into command building (as before) to first floor
2) once at the top of the stairs, turn right and go through the door (close it behind you)
3) turn around and face the door you just closed
4) strafe 3-5 feet left without turning
5) walk into the wall
6) turn ~80 degrees left
7) aim down sight
8) lean right (into the wall)
9) toggle the freelook (2x left alt, keep aiming down sights)
10) aim right (into the wall) until your camera clips through the wall
11) fire shots into the next room, you will be able to see the bullet impacts hit the wall in the next room
This takes the concept of the first glitch and takes it into the gamebreaking state, this clipping 'trick' will cause your gun to clip through the wall, and since your aim down sight is attached to your gun, your camera will clip through the wall as well. AI also accidently do this, since if they walk into a wall, their gun barrel (where i presume AI vision calculations are made, and where bullets are spawned into the gameworld when firing), can clip through and shoot people through walls and doors accidentally.
But wait, theres more (this one is the most egregious by far).
Same setup
1) enter command building, move to the first floor.
2) go prone
3) aim down sights
4) enable freelook (double tap left alt)
5) aim down
At this point, your player should be clipping through the floor, aiming straight down. from an outside perspective below, your player will have his head, torso and gun hanging through the ceiling, and from above, it will look like somebody cut your upper body off.