The Meaning of Life, the Universe, and Everything.
A couple of ideas I tossed together...
Crafted from Bricks, Gunpowder, and a String, the Pottery Charge is essentially an ancient hand grenade. The player must have a Flint & Steel in their inventory to use Pottery Charges. When thrown, the charge has a five second delay before emitting a deadly shrapnel-filled explosion. Unlike TNT, the explosion leaves surrounding blocks (except for Glass and Glass Panes) unaffected.
So the game's rating prevents guns from ever being added? How about an ancient pseudo-firearm? Crafted from a Bucket and two Sticks, the Fire Lance is a very effective weapon for dealing with mob swarms at close range. It uses Pottery Charges as ammunition and also requires a Flint & Steel in the player's inventory to function. When activated, there's a roughly one-second delay before the Fire Lance releases a wide spread of deadly shrapnel ahead of the player. The blast is faster and deadlier than a lone Pottery Charge, but is limited to close ranges and a more specified angle.