It also does nothing, go near the mouth when it is open after you have shot at the legs then it will say 'press 'X' to throw' obviously you should press 'X' then Chris will throw the grenade in its mouth and the top shell will fall off. What you are meant to do is get hand grenades (if you have none then look around the circle computer and you will find 2 herbs (green) and a hand grenade, DO NOT throw it at the beast.
When you are fighting the giant crab in chapter 5 then you probably know you have to shoot at the red tips on each leg, but when the monster comes crashing down and opens its mouth, shooting at it does absolutely nothing.
