how about a take-down 22lr rifle under one pound?

http://www.packrifle.com/

the weight savings in the rifle would allow you to carry extra ammo instead. at least with a 22lr you have more power and range.

and when used with CCI Subsonic HP #0056 it would be damn near silent too. in fact, i find #0056 in a rifle quieter than most pellet guns...