The .510 Whisper is a wildcat from SSK based on the .338 Lapua. It's designed for suppressed subsonic use and can fire a .50-caliber 750-grain A-MAX at 1,050 fps.
There's a very interesting hollowpoint bullet from Lehigh Defense which has large petals designed for maximum expansion. Strangely, the site claims these are supposed to have a .243 bullet inserted into the cavity. A similar or identical bullet is available loaded and weighs 725 grains.
Many also load cast lead bullets weighing up to 1,000 grains.