7mm RUM need ammo advice

[THERIDDLER07SMS]

Hello i have a remington 700 sendero chambered in 7mm Remington Ultra mag and wondering if anyone has a 7mm RUM with suggestions which ammo shoots the best. I am not reloading yet but will in the future. So for now that is not an option..

I have shot Remington 140gr ultra core lokt shells and Remington 150 Sirroco shells. both seem to shoot real well out to 300 yards. I want to know what ammo would be best for 600+yard shots.

Any help would be appreciated. thanks