Make a block spawn at an exact position

Hi, i want to make a weapon that spawn at an exact position, first, i made a block version of the weapon, when you right click on the weapon(block) , the block dissapear and the weapon appear in ur inventory, what i want is that he block spawn at x:20000 y:250 x:20000 (y is high but its because the block is affected by gravity). what i need to know, is how to make the block spawn when the world is first loaded, that mean, when minecraft generate the world for the time, the game generate the weapon(block) at x20000 y250 z20000. so when someone take the weapon, it will never spawn again.