The quiet yet powerful round of choice is the PMC moderator. It is the quietest 22 lr that will still function in a semi-automatic.
The Aquila SSS is a tadbit too loud unless you have a very long barrel. In some autoloaders the Aquila SSS will actually cycle and function.
Aside from that, head shots with CCI CB longs is an excellent choice.
Some of the Aquila Calibri rounds are very anemic in stopping power and accuracy.