Not enough power for hunting grouse. These are considered "parlour pistols", and were used commonly in France at the turn of the century in hotel parlours for target shooting. They are similar in power and accuracy to an air pistol. Even though they do not recommend adding black powder, I have seen some guys add 5 grains of 3F goex black powder for more velocity for longer range target shooting. Note, I have never done this and do not recommend it. Though it was interesting to see other guys do it at the range.
You need to use lead only projectiles, or else you will destroy the shallow rifling in them. So if you buy one, make sure you get some lead round balls, or modify the skirt (my rolling them between a ruler and a table) of a standard pellet to make ammo.
Hope this helps. They are small, but fit the hand comfortably. If you get one, you will have fun with it. even though it is a novelty item.