How bout a 22cal that shoots in the 800 to 850fps range thats about 7.5 to 8.5lbs thats ambi so everybody can enjoy the rifle with a similar set up like an HW97 or an AA Pro Sport!
Something like the Gamo CFX....But with a better lifespan. The breech seal needs to last, and a large breech for big hands/fingers.
I'm with the others on the idea of a Nitro Piston underlever of manageable weight and handling ability with integral Weaver/Picatinny rail.I would also add that it should be in the same power category as the NPSS, as well as of (crossing fingers) of U.S. manufacture.