Rain arrows in random positions within bounds? [duplicate]

setblock ~ ~1 ~ mob_spawner 0 replace {
    SpawnCount:1,
    SpawnRange:16,
    Delay:-1,
    MinSpawnDelay:1,
    MaxSpawnDelay:1,
    MaxNearbyEntities:32767,
    RequiredPlayerRange:32767,
    SpawnPotentials:[
        {
            Type:"Arrow",
            Weight:1,
            Properties:{
                Motion:[0.0,-1.0,0.0]
            }
        }
    ]
}

This will create a MobSpawner, spawning falling arrows at random in a square around it. The square is 3 high and 2*SpawnRange wide. On the current settings it will spawn one arrow per tick, you can increase SpawnCount to have more, or Min/MaxSpawnDelay to have less.

Of course this leaves us with a very derpy spawner up in the sky. The next step is to put it underground, and add a /fill clock that triggers a command block, teleporting arrows in the spawn square up in the air :

tp @e[type=arrow,x=...,y=...,z=...,dx=...,dy=...,dz=...] ~ 128 ~

What you're asking for can be done by creating a customized Monster Spawner. It can be customized to spawn things other than monsters, and to activate even if the players are far away.

The key thing a spawner gets you is randomized spawning locations.

Here's a command for a command block (it's too long for chat) which will create such a spawner:

/setblock ~ ~13 ~ minecraft:mob_spawner 0 destroy {EntityId:Arrow,SpawnData:{Motion:[0.0,-0.25,0.0]},SpawnCount:15,SpawnRange:15,Delay:0,MinSpawnDelay:1,MaxSpawnDelay:1,RequiredPlayerRange:15,MaxNearbyEntities:150}

The key things I learned while getting this to work:

• You must specify SpawnData for the arrows, or the arrows simply won't spawn at all.
• For some reason, the initial Delay value matters; when I had it higher than 0, it would only spawn arrows once, even though the MaxSpawnDelay was low.

Extra bits:

• The RequiredPlayerRange and location are set so that it'll activate when you're under it.
• The Motion makes the arrows be moving downward immediately, rather than appearing motionless and starting to fall.

If you want the rain of arrows to be controllable by redstone rather than player proximity, then set the RequiredPlayerRange to a large number, and use setblock commands to create or destroy the spawner as needed.

Make a clock with command blocks around it.

Set each block to summon an arrow in a specific position, and set up the specific positions to make it appear random.