restore_collection.js

// Connect to origin db
use some_database; // This will be named db

// Connecto to destiny db2
db2 = db.getSiblingDB('some_other_db');

// Only if needed clean destiny collection
db2.some_or_other_collection.remove({});

// Search all origin collection and copy each to destiny collection
db.some_collection.find()
  .forEach( function( data ) {
    db2.some_or_other_collection.insert(data);
  }
);

restore_collection.sh

$ mongodump -d some_database -c some_collection
$ mongorestore -d some_other_db -c some_or_other_collection dump/some_collection.bson