I know it has been told several times as well but.. I'd love to have an affordable AEG or GBB version of the Browning BAR M1918A1.
My second wish would be the Gepard M3 heavy self-loader sniper rifle But that I'd like to have with GBB only. (The RS one recoils)
http://en.wikipedia.org/wiki/Gepard_anti-materiel_rifles