This info comes from the book "The Pacific War Encyclopedia” by James F. Dunnigan and Albert A. Nofi:
“A true test would have pitted a squadron of Iowas against a squadron of Yamatos. The differences between the two classes of battleships are interesting. Since speed in a ship is partially a factor of hull length and fineness, her more powerful engines made Iowa about twenty percent faster than Yamato. In addition, Iowa’s deeper draft made for greater stability, making her a better “gun platform” than her rival. Both of these factors would have been important in an engagement at sea. Moreover, Iowa was a handier ship, responding to helm more rapidly, with a smaller tactical diameter (The minimum diameter necessary to make a full circle), which made her more maneuverable.
On paper, of course, Yamato’s thicker armor suggests a better protected ship, but this is rather deceptive. Thickness of plate must make an allowance for quality. In the years before World War Two, the U.S. Navy had made considerable strides in armor technology. As a result, the protection offered by its new armor plate was equivalent to about twenty five percent more thickness in terms of the older type of armor carried by the Yamato.
An additional important factor was that Iowa appears to have been better constructed than Yamato. On December 25, 1943, for example, Yamato took a torpedo that demonstrated that the jointing between the hull and her armor belt was faulty. As a full repair would have entailed the addition of over 5,000 tons to the ships displacement, the Imperial Navy merely patched up the damaged section, and pretended there was no problem. When a similar incident occurred to Musashi in March 1944, the Imperial Navy continued to pretend.
Of course the big difference was in the main batteries, Yamato’s 18.11 inch rifles (as battleship guns were traditionally called) making Iowa’s 16 inchers seem puny by comparison, what with the Japanese armor piercing shell weighing nearly twenty percent more and having almost seven percent more range. But this is a superficial comparison. A deeper look proves more interesting.
Gun caliber is given in inches and barrel length, so if a piece is described as 18.11”/45 the bore is 18.11 inches in diameter and barrel is 45 times that in length, or slightly less than 70 feet. The 16”/50 was about 66.6 feet long. Longer barrel length lends stability to the shell in flight, which increases range, which is one reason why, although the Japanese gun had a bore diameter thirteen percent greater than the American piece, its maximum range was only seven percent greater.
Other factors of importance were the propellant used (the “Gun powder”) and shell aerodynamics. On this score, the U.S. piece was better, with a slightly higher muzzle velocity (the speed with which the shell leaves the barrel, measured in feet per second, or fps).
Incidentally, the marginally greater maximum range of the Japanese gun would have been of no consequence. The greatest range at which a land-based heavy artillery piece ever hit a target deliberately aimed at appears to have been about 17.4 nautical miles (roughly 35,200 yards, 20 land miles) a feat accomplished by a U.S. Army model 16"/45 coast defense gun at Fort Weaver, Hawaii, in August 1938, under absolutely wonderful conditions of weather and sea. The longest range deliberate hit at sea in naval combat occurred on July 9, 1940, off Calabria, Italy, in the Mediterranean, when the British Battleship Warspite put a single 15 inch round into the Italian battleship Giulio Cesare at 26,000 yards, about 12.8 nautical miles.
It may seem odd to measure the penetrability of a gun at zero range (i.e., at the instant the projectile leaves the barrel), but it actually is of some value for comparison purposes. The number of rounds per minute that the pieces were capable of firing is a rather optimistic figure, since it was dangerous and exhausting to attempt to sustain maximum rates of fire for more than a few minutes. Of course, this still gave the U.S. gun a higher rate of fire.
In effect, on technical grounds, the U.S. 16”/50 battleship rifle was by no means inferior in performance to the Japanese 18.11”/45. And in action that performance would have been enhanced by fire control radar, a development with which the Japanese had very little success.
Below is the table for Main battery Gun Performance Comparison
Japanese 18.11”/45 American 16”/50
Muzzle velocity 2,559fps 2,600fps
0 yards 34” 32.62”
20,000 yds about 20.4” 20.04”
30,000 yds about 14.7” 14.97”
Rounds per minute 1 2"