The ‘Feared’ American Naval Gun That Outgunned Japanese Island Fortresses

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Today’s story is about a desperate problem American forces faced in the Pacific.

Japanese island fortresses that absorbed conventional naval bombardment and kept firing, threatening to turn the most ambitious amphibious assault of the war into a catastrophic failure.

The weapons that conventional naval doctrine relied upon, the very steel and fire that had won battles across the ocean, simply couldn’t crack these defenses.

This is how the Navy, facing a crisis of firepower, found their answer in the most unexpected place imaginable.

February 19th, 1945, the waters offima churned with the combined might of the US Nari Fifth Fleet, the largest amphibious assault force the Pacific had ever witnessed.

In hundreds of landing craft, American Marines stared at the desolate black sand beaches they were about to storm.

Beaches defended by 21,000 Japanese troops.

These soldiers were not waiting in shallow trenches.

They were entrenched in a subterranean nightmare of reinforced concrete bunkers, interconnected tunnels, and artillery positions carved directly into the island’s volcanic rock.

Looming over the entire scene was Mount Surabbachi.

Its slopes honeycombed with fortifications that 3 days of conventional naval bombardment had barely scratched.

That preliminary bombardment had been a bitter, smoke choked disappointment.

A powerful force of destroyers and cruisers had pounded the island with thousands of high explosive shells.

Yet the Japanese gun crews simply retreated deep into their labyrinthine tunnel systems, waited out the deafening barrage, then returned to their weapons moments after the shelling ceased.

For the men watching from the fleet, the spectacle was awesome, but the results were negligible.

The island’s defenses remained largely intact, ready to greet the Marines with a curtain of fire.

Admiral Richmond Kelly Turner, the overall commander of the amphibious operation, watched with growing frustration from the bridge of his flagship, UEISS Elorado.

He saw shell after shell from his cruiser screen detonate against Ewoima’s northern defenses.

Each explosion sent up spectacular fountains of black volcanic ash and pulverized rock, creating the powerful illusion of total destruction.

But Turner had seen the high alitude intelligence photographs taken just hours after previous bombardments.

The fortifications, like malevolent ghosts, reemerged from the smoke and dust, their structures seemingly untouched.

Japanese coastal defense guns remained operational.

Most chillingly, the underground networks that connected every defensive position showed no signs of collapse.

The problem was a matter of physics and flawed assumptions.

The 8-in cruiser shells, formidable weapons in a ship-to- ship engagement, couldn’t penetrate the incredibly thick concrete casements protecting Japanese coastal guns.

The heavy cruisers USS Pensacola and Indianapolis had dutifully fired hundreds of rounds at identified bunker positions.

Direct hits on the reinforced concrete produced spectacular starbursts of flame and shrapnel on the surface.

But when reconnaissance aircraft photographed the same positions hours later, the fortifications showed only minimal cosmetic damage.

The shells being used, primarily high capacity HC rounds, were designed with thin casings to maximize their explosive blast against surface targets like airfields or unarmored warships.

They lacked the sheer mass and penetrating power to crack through multiple meters of steel reinforced concrete that was in turn backed by dense volcanic rock.

The 5-in destroyer rounds were even less effective, merely cratering the blasted landscape without any hope of reaching the deeply buried positions.

Destroyers operating dangerously close to shore had pumped thousands of shells into suspected defensive works.

While these lighter projectiles were devastating against surface structures and any personnel caught in the open, they proved almost useless against hardened fortifications.

Photographic intelligence showed countless impact craters pocking the terrain around Japanese positions.

But the positions themselves remained stubbornly operational.

Even when direct hits were scored, the fortifications absorbed punishment that would have obliterated conventional structures.

In one particularly telling incident, a Baltimore class cruiser had scored three consecutive 8in hits on a concrete blockhouse protecting a 120 mm coastal gun.

Spotters confidently reported the structure as destroyed.

When Marines of the Fifth Amphibious Corps landed three days later and fought their way to that position, they discovered the block house entirely intact.

Its gun crew, having sheltered in deeply buried connecting tunnels during the bombardment, had returned to their weapon the moment the barrage lifted.

The concrete facing showed shallow, scooped out impact damage, but the structures integrity remained uncompromised.

Aboard the command ships, marine commanders grew increasingly alarmed.

Major General Harry Schmidt, commanding the fifth amphibious corps, studied the latest reconnaissance photographs, his face grim.

The images showed the designated landing zones covered by murderous interlocking fields of fire from positions that seemed completely impervious to mediumcur naval gunfire.

Every gully and every approach to the beaches lay under direct observation from elevated positions.

Japanese planners had expertly positioned their artillery to cover every meter of shoreline.

The photos revealed something even more disturbing.

The 3 days of intensive non-stop bombardment had failed to neutralize even a quarter of the identified defensive positions.

The Japanese had learned from the bloody lessons of previous island battles, building their defenses specifically to withstand the might of American naval bombardment.

After observing American tactics at Tarawa, Quuadilain and Saipan, Japanese engineers under the direction of their commander had redesigned their defensive doctrine entirely.

They had abandoned any serious attempt to defeat American forces at the waterline where overwhelming naval gunfire could effectively support the assault.

Instead, they constructed deep multi-layered defensive networks designed to survive the inevitable preliminary bombardment and emerge intact to contest the landing and bleed the invaders dry.

On Euima, Japanese construction battalions had excavated an underground fortress system unlike anything American forces had previously encountered.

Intelligence estimated over 11 mi of tunnels connecting thousands of defensive positions, command posts, and troop shelters.

The main artillery bunkers featured concrete walls 6 ft thick, constructed from steel reinforced concrete and buried under additional meters of volcanic rock and earth.

Many gun positions incorporated heavy steel doors that crews could slide shut during a bombardment, then reopen to engage the landing craft.

Command bunkers sat 20 to 30 ft underground, accessible only through cleverly concealed tunnel entrances scattered across the island.

Lieutenant General Tatamichi Kuribayashi, the brilliant and ruthless commander of the Japanese garrison, had explicitly designed these defenses to neutralize the core of American combat power, its naval firepower.

His defensive philosophy accepted that American warships would dominate the sea approaches, and deliver massive preliminary bombardments.

Rather than exposing his troops and precious artillery to this overwhelming force, he buried everything that conventional naval shells might destroy it.

planning to fight a brutal battle of attrition from below ground.

This strategy rendered American pre-invasion bombardment doctrine honed over two years of war almost obsolete.

The doctrine called for destroyers and cruisers to engage island defenses with sustained fire, destroying fortifications and neutralizing artillery before the marines ever hit the beach.

This approach had worked effectively where Japanese defenses relied on coconut log bunkers, light concrete pillboxes, and surface positions.

Against the subterranean fortress of Euoima, it failed completely.

Rear Admiral William Blandy, commanding the amphibious support force, had anticipated this.

He understood that cracking Euoima’s unique defenses required sustained methodical destruction of hardened targets, a task that demanded time.

No, he had recommended a 10-day preliminary bombardment.

Admiral Raymond Spruent, commanding the entire fifth fleet, rejected the request.

The complex operational schedules for the Pacific theater were relentless.

The Ewoima assault had to proceed quickly to free up naval assets, particularly the aircraft carriers, for the upcoming invasion of Okinawa.

It was a grim calculation of risk and resources.

Bland received authorization for just 3 days of bombardment.

Those three days consumed enormous quantities of ammunition while producing minimal results.

Heavy cruisers fired their main batteries until gun barrels showed dangerous wear.

Destroyers expended entire magazines of 5in shells.

Carrier aircraft dropped hundreds of tons of bombs.

The island disappeared under a paw of smoke and dust, giving the grand impression of total devastation.

But when the smoke cleared, the Japanese positions remained ominously operational.

Marine intelligence officers identified a particularly troublesome defensive complex on the northern end of the island.

Reconnaissance photos showed a cluster of heavily reinforced concrete bunkers protecting 150 mm howitzers and 120 mm coastal guns, all positioned to enilate the landing beaches.

These weapons could deliver devastating fire directly along the shoreline.

A steel scythe that would cut through the landing waves when the Marines were most vulnerable.

Neutralizing this complex became a priority target.

Six heavy cruisers concentrated their fire on the position over two days.

Observers reported multiple direct hits.

Aerial reconnaissance showed the area was heavily cratered.

Confident intelligence analysts marked the position as probably destroyed.

When exhausted Marines finally reached the complex days later after suffering horrendous casualties, they discovered all four artillery pieces fully operational.

Their crews had simply sheltered in the connected tunnels during the bombardment.

The cruiser shells had destroyed surface structures and killed Japanese troops caught in the open.

But the hardened gun positions themselves remained intact.

Something with far more devastating earthshattering power was needed.

The military required weapons capable of punching through meters of reinforced concrete and creating seismic shocks that could collapse the vital tunnel networks that smaller shells couldn’t reach.

Marine commanders desperately sought naval guns firing projectiles with enough sheer mass and velocity to crack open fortifications that absorbed 8in shells without critical damage.

The shells needed to be able to penetrate deep underground before detonating, collapsing the tunnels from within rather than merely cratering the surface.

Naval officers examined their available resources.

The fleet of modern cruisers currently bombarding Eoima represented the most powerful concentration of mediumcaliber naval firepower the Navy could deploy.

Yet against Eojima’s defenses, their shells were proving to be little more than irritants.

The answer, however, was already steaming toward Eoima, hulking silhouettes on the horizon that many strategists in the new carrier focused Navy had dismissed as relics of a bygone era.

These were the old battleships.

several of them survivors of the attack on Pearl Harbor, resurrected and refitted.

They carried weapons designed for a different kind of war entirely.

Yet, circumstances would prove their tremendous value in ways their original designers never imagined.

The ships had been built for fleet engagements against enemy battleships.

Their massive 14-in and 16-in guns intended to hurl armor-piercing projectiles through the thick steel belts of enemy warships at extreme range.

In an age of carrier aviation, many naval theorists considered them obsolete.

But those same characteristics that made them supposedly obsolete, their enormous projectile mass, their tremendous muzzle velocity, and their ability to fire shells designed to penetrate hardened steel armor made them potentially devastating against hardened shore fortifications.

A 16-in armor-piercing shell weighing over 1,900 lb and built to punch through 16 in of warship armor plate before detonating inside a ship’s vitals possessed exactly the kind of penetrating power needed against Eojima’s concrete and volcanic rock.

The challenge was one of mindset, recognizing that weapons designed for one purpose might prove invaluable for another.

American naval doctrine throughout the war had emphasized speed, mobility, and the primacy of carrier aviation.

The battleships represented the old navy, slow, fuel hungry, and requiring extensive support.

Many in the Navy’s high command, questioned their continued utility.

Yet here off Euoima, where modern cruisers firing modern shells against modern fortifications produced such disappointing results, those aging battleships carried exactly what the Marines on the beaches desperately needed.

Their guns could fire massive projectiles with enough velocity to penetrate targets that stopped 8-in cruiser shells cold.

The question was whether anyone would recognize what these old ships could accomplish before the Marines stormed beaches still covered by fully operational Japanese artillery.

As the preliminary bombardment continued to consume ammunition for little gain, the Japanese gun crews sheltered underground, waiting for the men in the landing craft.

Time was running out.

The solution and legacy.

The convoy that arrived at Ulithi Atal in January 1945 brought ships that looked like relics from another era.

But sailors aboard modern Essexclass carriers watched the old battleships steam past, their holes in dated camouflage, their superructures rebuilt from Pearl Harbor damage, but still bearing the architectural bones of the dreadnaugh age.

These were the old ladies of the Pacific Fleet, commissioned before World War I ended, kept in service while their modern sisters fought fleet actions.

What caught the eye, however, were the weapons dominating their decks.

Four massive turrets, each holding three 50- FFT gun barrels weighing nearly 80 tons a piece.

These turrets rotated with surprising smoothness.

Their complex internal mechanisms of shell hoists, powder trunks, and massive breach blocks resembling industrial factories more than weapon systems.

high at top rebuilt towers.

Num fire control directors housed huge optical rangefinders and multi-tonon mechanical computers.

The new amphibious force commander who came aboard USS Tennessee studied these weapons with undisguised skepticism.

The ship’s gunnery officer, a veteran, demonstrated the loading sequence.

Sir, each gun can fire two rounds per minute.

That’s 24 from all four turrets.

The commander, comparing this to the destroyers and cruisers that had pounded Tarawa, remained doubtful.

“Those fortifications are reinforced concrete,” he said.

“We’ve hit them for days.

What makes these different?” The officer gestured to a 14-in armor-piercing shell standing 5t tall.

“Wait, sir.

These shells weigh, 1560 lb, propelled by a 425lb powder charge at 2,600 ft per second.

But that projectile carries kinetic energy nothing on those islands can withstand.

The weapon was the 14in/50 caliber MarkV naval gun designed in 1912 for the US Navy’s New Mexico and Pennsylvania class battleships.

The 50 caliber designation meant the barrel’s 50ft length was 50 times its 14in bore diameter.

It could hurl two types of projectiles over 17 mi.

a 1,560lb armor-piercing AP shell and a 1,400 lb high-capacity HC shell.

The guns were mounted in 800 ton triple turrets protected by 18 in of face armor.

This configuration represented a pre-war design compromise, sacrificing the larger 16-in caliber of later battleships to mount 12 guns instead of nine, maximizing the rate of sustained fire.

The design reflected preWorld War I doctrine where battleships were expected to engage each other at long range using plunging AP shells against deck armor.

The 14-in/50 provided sufficient power and a high volume of fire with classes like the Pennsylvania and New Mexico carrying 12 guns in four triple turrets.

By Pearl Harbor, however, these ships were obsolete for fleet actions against newer enemy battleships with larger guns.

American doctrine thus relegated the old 14-inch ships to convoy escort and shore bombardment while the new 16-inch Iowa and South Dakota classes hunted the enemy fleet.

Shore bombardment, however, revealed unexpected advantages.

After the bloody assault on Tarawa in November 1943 exposed the inadequacy of lighter naval guns, the Navy re-evaluated its oldest battleships.

The test came at Quadrilene in February 1944.

You know, where USS Tennessee and USS Pennsylvania joined the bombardment? Fire control officers made a critical discovery.

The AP shells designed to pierce warship armor were devastating against concrete fortifications.

They punched through reinforced concrete that stopped smaller shells, penetrated deep into bunkers, and then detonated.

The key was the 0.033 033 second delayed action fuse which allowed the shell to travel 15 to 20 ft inside the target before exploding.

A captured Japanese engineer confirmed the effect, describing how single 14-in hits destroyed casemates that had withtood days of bombardment, killing the entire garrison.

Ioima represented the supreme test.

The island’s defenses were the most formidable yet encountered.

11 miles of tunnels connecting 1,500 cave positions and pill boxes built into volcanic rock, many invisible from the sea.

Preliminary bombardment began on February 16th, 1945 with USS Tennessee, USS Idaho, and USS New York, taking positions just 3,000 yards offshore.

The plan was a systematic destruction of known positions.

Tennessee’s gunnery officer, for example, divided Mount Suraki’s northern face into 32 target zones and methodically pounded each with 6 to 12 AP shells.

The results exceeded expectations.

Marine Lieutenant Colonel Donald Weller watched as a Tennessee shell struck a concrete blockhouse.

The entire structure vanished in the explosion, he wrote.

No rubble remained visible.

The position had been completely destroyed.

During the campaign, Tennessee expended 2,241 rounds of 14inch ammunition with Idaho and New York adding over 3,500 rounds combined.

This sustained fire was possible because the old battleships carried more ammunition than their modern counterparts.

Tennessee’s magazines held 1,350 rounds, significantly more than the 1,080 rounds carried by the larger gunned Iowa class, allowing them to remain on station, providing fire support for longer.

The shells demonstrated unique tactical advantages.

Those penetrating tunnel entrances could collapse entire underground complexes with Japanese survivors describing the impacts as earthquakes that buried defenders.

The heavy shells also proved effective against targets in defilade positions on reverse slopes invisible to direct fire.

The gun’s maximum elevation allowed for plunging fire, dropping shells at steep angles onto the tops of bunkers.

This firepower could be delivered with surprising precision during critical counterattacks.

Tennessee and Idaho provided danger close fire support, placing 14-in shells within 300 yd of marine positions.

This was feasible thanks to the accuracy of the Mark 34 fire control director and its rangekeeper mechanical computer which could place shots within 50 yards at a range of 10,000 yd.

The communication chain from spotters ashore to the battleships allowed for shells to be on target in under 3 minutes.

Okinawa demonstrated the weapons sheer endurance throughout the 82-day campaign.

The old battleships provided continuous fire support.

Tennessee alone fired 3,846 rounds, requiring multiple replenishments at sea.

The gun systematically reduced Shuri Castle, the ancient fortress anchoring Japan’s defensive line, its 3-fft thick limestone walls shattering like pottery, and gunnery logs show the methodical destruction of key positions like Conicle Hill and Sugarloaf Hill, fortified villages built into reverse slopes that were vulnerable only to the plunging trajectory of heavy naval guns.

Post battle surveys confirmed that 14-inch shells had penetrated stone, earth, and concrete that had stopped all other forms of ordinance.

The weapons were also unexpectedly reliable.

Though designed for 300 round barrel life, combat necessity pushed that to 400 rounds or more with crews rellining barrels between campaigns.

Tennessee’s number two gun in turret 1 fired 6,127 rounds during the war.

The main weakness was the deterioration of the 425 lb silk powder bags in tropical conditions which could affect muzzle velocity, but gunners compensated by firing ranging salvos.

The battleships were also able to counter every Japanese tactical adjustment.

When defenders moved, the ships fired interdiction barges.

Their heavy armor made them nearly immune to counter battery fire.

A 150 millimeter howitzer shell that struck Tennessee’s Turret 2 at Okinawa merely left a dent for nighttime attacks.

The battleships delivered pre-registered time on target bargages that saturated assembly areas.

Prisoners reported the distinctive freight train shriek of incoming shells caused panic even among veteran troops.

Despite their power, the guns had significant limitations.

The slow rate of fire, two rounds per minute per gun, could not provide the suppressive fire of destroyers rapidfiring 5-in guns.

They excelled at destroying hardened targets, but were inefficient against dispersed infantry.

The cost and logistics were also considerable with each shell costing $1,350 in 1945 and requiring specialized replenishment.

Most critically, the weapons were indiscriminate.

Several friendly fire incidents occurred, such as when a short round from Idaho killed seven soldiers at Okinawa.

The same delayed action fuse, so effective against bunkers, was a hazard near friendly troops in soft ground, burying itself deep before creating massive craters.

Production of the MarkV gun itself ended in 1920.

Seven battleships across the Pennsylvania, New Mexico, and Tennessee classes were equipped with these 12 gun main batteries.

Wartime industrial focus shifted entirely to ammunition.

From 1943 to 1945, naval arsenals produced 47,14in projectiles.

However, production peaked at 2,100 rounds per month in early 1945, a rate that could not keep up with combat expenditure.

By the end of the Okinawa campaign, ammunition shortages began to restrict fire missions to only the highest priority targets.

After Japan’s surrender, the old battleships were rapidly decommissioned.

Though briefly considered for recall during the Korean War, none of the MarkV equipped ships returned to service.

Their stockpiled ammunition, however, was used to support other operations.

A modern military assessment confirms what the Navy discovered in 1944.

The 14-in/50 Mark IV was an ideal shore bombardment weapon.

As noted by naval historian Robert Ballard, while 16-in guns had more power per shell, the 14-in guns combination of a sustainable fire rate, higher ammunition capacity, and superb penetration against concrete made them more effective for this specific fire support role.

The ability to deliver a 1,560lb projectile with a delayed action fuse was the perfect tool for destroying any fortification the Marines encountered.

Tragically, none of the battleships mounting these historic guns, such as the USS Tennessee, were preserved as museum ships.

All were eventually scrapped.

The problem that faced Marines on those volcanic islands found its solution in weapons built three decades earlier for an entirely different war.

The 14-in/50 MarkV naval gun, designed for fleet actions that never happened, instead became the weapon that made amphibious assaults survivable.

It turned impregnable fortresses into rubble and gave American forces the decisive firepower that saved countless lives and overwhelmed Japan’s strategy of defense in depth.

With over 23,000 rounds fired, the legacy of these old guns was cemented.

The empire that built those island fortresses learned too late that raw, concentrated, explosive power, properly applied, never truly becomes obsolete.