It's # 5 in the picture.  I am considering making up a carbine that will look somewhat like an XM-177, but want to keep the 16" barrel length.  Has anybody ever used one of these?  Have a pic to share?  Would it work with a mid-length upper setup?  Inquiring minds want to know.

TIA for your replies..

Heavy and reduces cooling ability.  But it looks kewl, I guess.

This is Bushmaster's reverse flash suppressor on their 16" superlight weight barrel.

It is a heavy chunk of steel. Rather amazing how heavy it is. But that said the rifle balances at the slip ring with it on and at the pivot pin with a birdcage/Phantom flash suppressor. There is a subtle difference in how the rifle handles between the two. I really can't decide which way I like it, but I've shot the gun more with the reverse f/s on than not.

Why?