If I remember correctly, the old 150 grain flat-based .30/06 load at 2,700 fps. that the Army used before and during WW1 had a range of 3,500 yards. By going to what they called the M-1 load (not to be confused with the M-1 rifle), which featured a 172 grain boat-tailed bullet at 2,660 fps, the range was extended to 5,500 yards. That demonstrates the effectiveness of a boat-tail in helping to increase ballistic efficiency at extreme ranges, long after the velocity has dropped to sub-sonic.
The Army later dropped that load in favor of the M-2 load which featured a 152 grain flat-based bullet at about 2,800 fps. That's the load we used in the M-1 rifle during WW2 and Korea.
I got most of the information above from page 109 of Jack O'Connor's book entitled "The Hunting Rifle".