Ruby ampersand colon shortcut

Your question is wrong, so to speak. What's happening here isn't "ampersand and colon", it's "ampersand and object". The colon in this case is for the symbol. So, there's & and there's :foo.

The & calls to_proc on the object, and passes it as a block to the method. In Ruby, to_proc is implemented on Symbol, so that these two calls are equivalent:

something {|i| i.foo }
something(&:foo)

So, to sum up: & calls to_proc on the object and passes it as a block to the method, and Ruby implements to_proc on Symbol.

There's nothing special about the combination of the ampersand and the symbol. Here's an example that (ab)uses the regex:

class Regexp
  def to_proc
    ->(str) { self =~ str ; $1 }
  end
end
%w(station nation information).map &/(.*)ion/

=> ["stat", "nat", "informat"]

Or integers.

class Integer
  def to_proc
    ->(arr) { arr[self] }
  end
end

arr = [[*3..7],[*14..27],[*?a..?z]]
arr.map &4
=> [7, 18, "e"]

Who needs arr.map(&:fifth) when you have arr.map &4?