Up until the 1920s, every caliber of gun in USN service had its own "Mark" numeric series along with an additional "Mark" numeric series for the mountings used for that particular caliber. So, for example, for the famous 5"/38 of World War II, the "Mark 12" designation was for the gun itself. The mountings used for this gun were Mark 21 for the earliest pedestal mountings, Mark 24 for later pedestal mountings, Mark 25 for the prototype single base ring mountings, Mark 30 for the production single base ring mountings, along with several mark numbers for the twin mountings, ending with the Mark 38 used on the AM Sumner and Gearing destroyers.
Larger caliber guns originally had a similar designation series for guns and mountings, but guns 6" and larger developed after the late-1920s did not get Mark numbers for their mountings. Instead, they were now simply known by the class of ship that they were used on. For example, the 16" guns used on the Iowa class battleships were designated as Mark 7, but the mountings themselves were simply described as "Iowa class 16-in three-gun mountings."
During the early 1940s, the USN developed the 5"/54 which was first used on the Midway (CVB-41) class carriers. The gun barrel for this weapon was designated as Mark 16 and the single mounting used on the Midway class was the Mark 39 (the next 5 inch mounting number after the Mark 38 used on the AM Sumner and Gearing destroyers). The Mark 40 mounting was a special "wet-mount" version of the older 5"/25 for submarines. The Mark 41 was a twin mount intended for the cancelled Montana (BB-67) class battleships and was to have used the same 5"/54 Mark 16 gun as used on the Midways.
No new guns larger than 3" were developed until the early 1950s, when a new 5"/54 was developed for destroyer-sized vessels. At this point, the USN decided to change their designation system. New weapons would now no longer be known by the gun barrel mark designation, instead, they would now be known by the mounting designation. So, this new 5"/54 mounting was designated as "Mark 42" and it was used on numerous destroyers, cruisers and carriers as well as on auxiliary vessels (by the way, the new 5"/54 gun itself for this mounting was designated as the Mark 18, a continuation of the 5" gun designation series).
In another change in designations that occurred at about the same time, the USN decided that there would no longer be a separate mounting mark numeric series for each caliber of gun. Instead, all shipborne guns would henceforth be lumped together into a single numeric mark series, which would continue from the Mark 42. Notable examples in this series are the 8" Major Caliber Light Weight Gun (MCLWG) installed on USS Hull (DD-945), which was the Mark 71; the famous Phalanx CIWS is the Mark 72 (Mark 15 is the gun system); and the 76 mm OTO-Melara gun is the Mark 75. Almost all naval guns are now in this series, but some weapons are more commonly known by their gun system designations. For example, for the 25 mm "Chain Gun," Mark 38 is the designation of the gun system, not the mounting. The normal non-stabilized mounting used for the Chain Gun is designated as the Mark 88. The stabilized mounting with an integral 40 mm grenade launcher as used on the Cyclone class patrol boats is designated as the Mark 96. Again, these mounting designations are in the standard naval gun mounting series.
This designation-by-mounting system has continued until the present day with some Mark numbers being given to weapons that were not accepted into service and several others being given to minor modifications of existing mountings. An example of this latter type is the Mark 84, which was a modified mounting used for the three-barrel 20 mm M197 Gatling gun.
One of the latest examples of the current designation system is the Bofors/United Defense/BAE 57 mm/70 Mark 3, which the USN has designated as the "Mk 110 Mod 0/57 mm" gun. This weapon will enter service on the new Coast Guard Maritime Security Cutters (Large) and will also be used on the Littoral Combat Ships (LCS) and the Zumwalt class guided missile destroyers (DDG-1000).
An interesting exception to the designation system is the 30 mm Bushmaster II as used on the San Antonio class LPDs. The gun itself is designated as the Mark 44 Mod 1 while the mounting is the Mark 46 Mod 1. This mounting designation is actually in the Marine Corps' designation system, not the Naval one. The mounting for the San Antonio class is almost the same turret as used on the Expeditionary Fighting Vehicle (EFV), formerly known as the Advanced Amphibious Assault Vehicle (AAAV). The Mark 46 Mod 0 is the EFV turret while the Mod 1 is a Navalized version used on the San Antonio LPDs. This simplifies the logistics system by allowing this class of ships to use the same ammunition and spare parts as used on the EFV.