hrmh like i already said, the grenade model is stored in frag_grenade.chr
the way the frag grenade works, is that you have the model in one file, then you have another file with a rig, which has an animation. this is player.chr that came with the example files for c2 mod SDK. the animations are stored in animations>human>male>weapons>frag_gren
if you just want to replace the model, you dont need to worry about the animations. The point of having the animations seperate from the model, is that in the engine the game will just paste the model on to a bone in the animation, meaning you can change the model to whatever you want without losing the anims.
edit: there is no grenade in sample assets, but if you just overwrite in the matching directory in your mod dir, it will use your custom model instead of he vanilla one.