Battlestate, developer of FPS Escape from Tarkov, has released a trailer that shows off the extensive and detailed weapon customisation in the game.
In the trailer and M4 is used to show all the different aspects of the gun that can be customised. It also shows other weapons being broken down and then rebuilt, with things like magazines and sights featuring in the options available.
Escape from Tarkov is currently available on PC in alpha state.