For years I've taken Lambert's Warrior at its word, the initial velocity of a 68 pounder was ca. 1,580 fps. Now, reading contemporary works from the 1860's I find that this is not the velocity with a "far" or "battering" 16 lb charge, but seems to be the velocity with the 12 lb "service" charge.
Reading Holley's Treatise on Ordnance and Armour I find that with the 16 lb charge the gun developed a muzzle velocity of over 2,040 fps. This goes an awfully long way to explain why the Royal Navy kept this gun as an anti-armour piece into the mid-1860's. For comparison, with a 60 lb charge (of Mammoth) and a 451 lb ball a 15" Rodman only developed around 1,200 fps.
The key parameter to consider is the powder charge: shot weight ratio. In the 15" even with 60 lbs of powder the powder charge is only 13% the shot weight. In the 68 pdr it is around 25%. The slow burning Mammoth is less efficient, and the shorter barrel (in calibres) means the burning charge has less time to accelerate the projectile.
Simply correcting myself after years of using the 1,580 fps figure....