What is this spring loaded pin for

Just doing some things to my ACR and wondered what this spring loaded pin and mounting block is for?

Obviously for some front rail/grip mount but anyone know which one? It’s a lot of engineering for nothing…

That is where the hidden gubamint tracking device is hidden… laughing%20(1)


The spring loaded pin is to lock the outer barrel collar in place so it keeps the t-piece from moving forwards. If you press the locking pin in and turn the black plastic piece above it you will find it rotates and releases.