I use the Eberlestock Just One but my rig is just an accurized Ruger M77 MKII 300 Win Mag.
If you want a pack that will carry a rifle that large, look at the Gunslinger II. My friend bought one as it was the only pack he could find from Eberlestock that would fit his Savage 110 BA 300 Win Mag w/ a nightforce nxs 5.5-22x50.
2013 Eberlestock Gunslinger II Pack
Even though he was only lugging that cannon around the hills in Eastern MT, it got heavy quick!