True, but a quarter pound lead ball would take up less room. I'm sticking to my guess of low to mid 2000's. Ask him what kind of velocity he's getting if you could. Demanding gun nerds want to know.
Just for fun, I ran some numbers through a free recoil energy calculator. Assuming a rifle weight of 15lbs, a projectile weight of 2000gr and a velocity of 1800 ft per second, I arived at 615 ft lbs of energy. Thats over 14 times the energy created by a 30lb 50 bmg. Yowza. Might not move the Missouri several feet, but short of a vehicle mount, I wouldn't fire it.