For as quiet as you can get, go with the Colibri primer only rounds. They use a 20 grain cone bullet. It may be difficult to find locally, but here's a couple sources:

http://www.cheaperthandirt.com/AMM066-3528-304.html
http://www.midwayusa.com/eproductpag...eitemid=295321

It will do a rabbit in at 25 yds. They work best from a revolver, but shoot fine through my Remington bolt action rifle. They won't cycle a semi-auto action. But quiet they are.