If You don`t want a custom rifle You might be interested in the Merkel K1 rifle. The K1 has an action made from aluminium alloy and weighs about 2,5 kg. The action is the excellent "Simson-Jäger-Verschluss" which is much stronger than anything else I know except a falling block action. Blaser uses the same action in their K95, which is an alternative but only if You are not interested in craftmanship, all Blasers are completely made on CNC-machines.
If You want something special, have a look at a Martin Hagn take-down falling block rifle.

Kalunga