I have a sketch done, but not the actual picture. What I did was I did a quick Firefight run, chose the grenade launcher and fired a few rounds, saved and quit. I then went into the Theater and watched in slow motion as I reloaded. The grenade round that is fired is one in the same, meaning there is no casing for the round itself, like most 40mm rounds. I watched it in first person and third person. Hope this helps.
I also wrote down some potential dimensions on it too.