Although the BRRT is listed as a toggle, it doesn't show visually on the model. So this small addition could be a subtle way to physically show that the attachment is on & working. I figure, since that part I circled is exactly the same as the one next to it, it should have some kind of function instead of being vestigial. And the BRRT attachment might be the perfect thing to give it a purpose.
Suppressor should totally be a barrel attachment for the MCG. It will be basically useless but hey, why not?
Well, it being a rail attachment is ********. I can see why people say that...MCG needs Exmag in that slot. BRRT should be rail or anywhere else but where it is.
