Bilbo saw that the moment had come when he must do something. He could not get up at the brutes and he had nothing to shoot with; but looking about he saw that in this place there were many stones lying in what appeared to be a now dry little watercourse. Bilbo was a pretty fair shot with a stone, and it did not take him long to find a nice smooth egg-shaped one that fitted his hand cosily.
As a boy he used to practise throwing stones at things, until rabbits and squirrels, and even birds, got out of his way as quick as lightning if they saw him stoop; and even grownup he had still spent a deal of his time at quoits, dart-throwing, shooting at the wand, bowls, ninepins and other quiet games of the aiming and throwing sort-indeed he could do lots of things, besides blowing smoke-rings, asking riddles and cooking,
that I havent had time to tell you about. There is no time now. While he was picking up stones, the spider had reached Bombur, and soon he would have been dead. At that moment Bilbo threw. The stone struck the spider plunk on the head, and it dropped senseless off the tree, flop to the ground, with all its legs curled up.
Just as Roman slingers graduated from stones to ceramic and lead bullets, so would mostly likely have modern hobbits. 20ga is about the right bore for most halflings, though some of the more stout specimens rock a 12 bore.