Hiding in hidden rooms on every airfields that you cant enter etc, and you see everything around it and shoot over the wall when u go close to wall with ur weapon then it shoots by the wall:

Here is a video of it on airfield building for example:

https://www.youtube.com/watch?v=TjALTDH__VM

HOW TO ENTER SUCH ROOM:

1. TAKE YOUR WEAPON TO YOUR HAND AND PRESS SPACE

2. PRESS W AND RUSH THE DOORS AND BEFORE DOORS LIKE ONE METER KEEP SPAM 'V' BUTTOM AND KEEP PRESSING W BUTTOM AT THE SAME MOMENT AND THEN YOU ARE IN.

HOW TO LEAVE SUCH ROOM:

1. Same as entering, put ur wep to hand, press space.

2. Rush a table which is in a room next to the 'hidden' one usually left from the doors and do the same as in step 2 as in entering, go with W and while one meter before table keep spaming 'V' buttom and hold your 'W' buttom as well and you are out.