I guess nobody knows.

I think it looks pretty cool though

It looks like ammo or a hand rest.

I think the 'it just looks cool' thing might be right.

I thought maybe it's for resting his rifle on but I can't seem to do the bodily contortions to make that seem possible.

If it was on my armour I'd be constantly worrying about catching it on something.

Maybe it can be removed and use as a weapon but I think he already has a knife so that might be unlikely.

It’s a hand rest for when he needs to rest while shooting. I believe most snipers have them

It was used during TCW episode 'Unfinished Business' when Crosshair rests his rifle on it to make a shot.

^ Okay, nice. Thanks for that. Must have missed it. Sorted.