Speaking of long range shooting with doubles, my 450/400 3 1/4" Nitro Express Magnum has a fixed sight marked 50 yards and folding leaves marked 100,200,300 and 400 yards.
The left barrel is marked "THE SIGHTS ARE ACCURATE FOR THIS BARREL TO 400 YARDS"
The right barrel is marked "THIS BARREL IS ACCURATE WITH LEFT TO 50 YARDS".
I have shot the rifle on my 200 yard rock with good results. I killed a caribou with it at 150 to 175 yards. I guess I need to shoot it on paper at 300 and 400 yards to test out the makers statement.