Hmmm I think you're refering to what's just the 'extra armour' guys. It's mdeled to resemble a 40K version of ablative or reactive armour. the grenade launchers are just what they are, grenade launchers resembling current day smoke launchers on tanks.
Image:WH40k LandRaiderCrusader unpainted.jpg - Wikipedia, the free encyclopedia
Anyways, seriously cool progress, loving the assault cannons.