Sorry, I put the wrong picture up
. I meant for this one to be there instead:
It's a flash suppressor. The sound suppressor already takes care of the recoil issue associated with the large .338 round and cuts down on muzzle flash a bit, but the add-on flash hider goes further. Typically a suppressor fills the role of a muzzle break since it's slowing down the round and expelling energy prior to the bullet leaving the barrel, as a result they also lower the bullets muzzle velocity, noise profile and flash as well.
This is something rifles are now starting to see as attachment on or added into sound suppressors
Here's the same rifle as the .50 Barret that I accidentally put in my post, but from a different angle.
That's also a flash hider integrated into a sound suppressor. There's an MRAD on the far left of the picture.