Revealing mistake: When Ryback busts out his fellow mates and hands out weapons to them, the Uzi given to one of his mates does not have a magazine in it. (01:02:20)
Revealing mistake: When Ryback jury-rigs a timed explosive to destroy the helicopter, he uses a tin of paint thinner which is expected to fall off a hand grenade, thus releasing the spring on the grenade. However, when the explosion occurs you can see over Ryback's shoulder that the paint thinner tin hasn't moved at all, and that the explosion occurs inside the helicopter, not where the grenade was placed.
Revealing mistake: At the part where Ryback places a grenade on the door and the man gets blown away by opening it, the shot just before reveals that it's an obvious dummy (best seen in slow-motion).
Revealing mistake: When Seagal kills Tommy Lee Jones, he either sticks the knife into his head or smacks him on the top of his head with his fist (it's not that clear). But what appears to be clear is that this is in fact a dummy head or a stunt double - it looks very unlike Lee Jones.
Answer: The smaller guns were only equipped with the Starburst rounds. The 16 Inch Cannons were still supplied with live ammo. None of the characters left to fight had experience with the 16 inch guns except for the Gunner's Mate, therefore none of them thought to use one of the 16 inch guns.
dablues7