In volume II of Col. Biemick's Encyclopedia of Black Powder5 Artillery Projectiles, the 9pdrs are all over 4" in diameter. He writes on page 430 that "there is no direct evidence that the French produced a shell in this (8pdr) caliber, but it likely existed" and "It was likely 3.99" in diameter." It seems a bit large to be a grenade. I didn't see any that big in a brief look at his Volume IV which covers grenades.