The Mark 42 5-inch naval gun is a U.S.-origin 127 mm automatic shipboard mount that paired a 5-in/54 Mark 18 gun with the Mark 42 mounting. It served on U.S. destroyers, cruisers, frigates, and aircraft carriers and is directly documented firing from U.S. Navy destroyers during the 1955 Vietnam War.
U.S. Navy destroyers used Mark 42-equipped 5-inch guns for Vietnam naval gunfire: Turner Joy was photographed firing a 5/54 Mark 42 mount at an enemy shore position on 20 June 1968, and Buchanan was photographed shelling targets in Vietnam in 1968.
The Mark 42 combined a single 5-inch/54 gun with a complex automatic mount. Its loading path began below deck and alternated rounds through right and left cradles, giving the mount its high two-sided rate of fire while retaining a reduced-rate single-side fallback.
Mark 42 is the mount designation; NavWeaps identifies the gun itself as the 5-in/54 Mark 18.
Source: NavWeaps: USA 5/54 Mark 42.
The Navy training text describes remote control through the Mk 68 gun fire-control system as normal, with local control possible from the One Man Control station.
Source: TPub Seaman Manual: 5/54 Mk 42 Gun.
Tin Can Sailors describes two rotating loaders, lower hoists, transfer stations, upper hoists, cradles, transfer trays, and fuze setters feeding the single gun.
Source: Tin Can Sailors: 5/54 Single Mount.
Each rotating loader used projectile and powder-case drums; each drum held twenty rounds and could be replenished through loading doors.
Source: Tin Can Sailors: 5/54 Single Mount.
The manual describes two almost separate loading sides; a single side could continue at 17 rounds per minute if the other side was isolated.
Source: TPub Seaman Manual: 5/54 Mk 42 Gun.
NavWeaps notes a Vietnam-era reputation for jamming during prolonged firing, followed by rate reductions and later upgrades.
Source: NavWeaps: USA 5/54 Mark 42.
Mark 42 is the mount designation; the barrel itself is the 5-in/54 Mark 18 gun. Later service references distinguish upgraded Mod 9 and Mod 10 mounts from earlier retired versions.
| Variant | Configuration | Designation notes |
|---|---|---|
| Mark 42 Mod 9 | Late-service automatic mount | The Navy training text says the Mod 9/10 5-in/54 gun remained in fleet use, could engage surface, shore, and air targets, and fired about 34 rounds per minute. Sources: TPub Seaman Manual: 5/54 Mk 42 Gun, Archive.org: Combat Systems And Weapons Department Management |
| Mark 42 Mod 10 | Late-service automatic mount | GlobalSecurity describes the Mk 42 Mod 10 as a two-part mount with below-deck carrier, hoist, and ammunition-loader drum components feeding the upper hoist, trays, rammer, and firing components. |
The Mark 42 and Mark 45 are separate 5-inch naval gun families, but the later Mark 45 is documented by the U.S. Navy as the lighter replacement path for the Mark 42 mount.
| Compatible item | Item type | Compatibility evidence |
|---|---|---|
 | Successor 5-inch naval gun mount | The Navy Mk 45 fact file identifies the Mark 45 as the replacement developed for easier maintenance and lower weight than the Mark 42. |
NavWeaps dates the design to about 1950 and describes the Mark 42 as the post-World War II replacement path for older U.S. 5-inch mounts.
Sources: NavWeaps: USA 5/54 Mark 42
NavWeaps lists 1954 as the in-service date and identifies USS Northampton as the first ship class application.
Sources: NavWeaps: USA 5/54 Mark 42
The Wikimedia/NHHC caption identifies USS Turner Joy firing one of her 5/54 Mark 42 mounts toward an enemy shore position in Vietnam.
Sources: Wikimedia Commons: Turner Joy Mark 42 firing
The U.S. Navy Mk 45 fact file says the Mk 45 was developed as a lighter, easier-maintained replacement for the Mk 42 mount.
Sources: U.S. Navy: MK 45 - 5-inch 54/62 Caliber Guns
