Battlefield Archaeology: The massive 340 mm naval guns protecting Keroman U-Boat base at Lorient

WW2 in Greece

By Pierre Kosmidis

Research by Evan Panagopoulos, used by permission

All colour photos are © explorabiliaused by permission

Evan Panagopoulos is an avid fan of brutalist and mid-century architecture, a knowledgeable WW2 and Victorian-era enthusiast, and likes engaging with abandoned spaces and obscure history.

Evan Panagopoulos  is the urban storyteller behind the alternative travel site explorabilia and the Abandoned Sector blog.

Evan has a knack for rediscovering forgotten and unseen spaces hiding in plain sight, and expresses what he’s passionate about through writing, photography and interviewing people with a fascinating story to share.

In the early 20th century, the French equipped their surface fleet with one of the most unique and formidable weapons in naval history : this is the fascinating story of the 340mm/45 Modèle 1912 gun, which saw service in two wars, both on land and at sea, whether in turrets, emplaced, cradled, or on rails.

Below is a video of a French railroad gun exiting the cannon foundry at Ruelle-Sur-Touvre before the war (probably in the late 30s).

It shows a Canon 380 mm / 45 model 1935 (yes – this was an even bigger naval gun than the 340mm, but that’s another story) mounted on what appears to be a St. Chamond à berceau cradle.

This is as close as it gets to what the ones based at at Baterie Plouharnel would have looked.

And in a dramatic turn of events, some of the 340s France built even saw action against each other during the final stages of WW2.
Shortly after the Fall of France in June 1940, the Germans begin to build a number of fortified U-boot bases along the western Atlantic coast.

The largest two of those bases, comprising 44 submarine pens to shelter 4 flotillas, were built between 1940 and 1942 in Lorient, and St.Nazaire.


And between these, at the neck of the Quiberon peninsula, close to the small Breton village of Plouharnel, ample space was cleared to create a formidable battery.

It was considered to be one of the most powerful artillery sites in the Atlantic Wall, and its unique guns have a fascinating story.

With a firing range of 38km, these 340mm coastal guns would be capable to interdict a huge area off the coast of Bretagne, denying naval access to the Gulf of Morbihan, and more importantly, protecting the Keroman Submarine Base at Lorient.

Their arc of fire could also link up with that of the 240mm guns battery at Batz some 60km down the coast, which in turn protected the nearby St. Nazaire Submarine Base from seaborne intruders.

For comparison, the coastal guns at Plouharnel boasted a longer range than the Bismarck, or any ship of the Royal Navy at the time.

In fact, they had about the same range as those of a US Iowa class battleship, and only slightly inferior range than those of the Japanese Yamato class battleship : and these were perhaps among the heavier naval guns that ever saw action!

This naval comparison is not entirely coincidental. The coastal guns at Plouharnel actually emerged in the service of the French Navy circa 1912.

The early 20th century was a monumental period for naval warfare, a time when the construction of ever heavier battleships by the powerful navies of the era resulted in a new class of warship, the so called Super-Dreadnoughts. It all began with HMS Dreadnought, a heavy British battleship launched in 1906.

The enormous firepower she carried instantly tipped the balance of power at sea, and prompted a naval race without precedent among nations of the time.

The French response to this event was the Bretagne class of battleships, a series of three Super-Dreadnoughts equipped with the 340mm/45 Modèle 1912 naval gun in 4x or 5x twin turret configuration as main armament – the type of barrel that was eventually stationed at Plouharnel.

The evolution of that class was to be the MF Normandie, first in a new ship class designed to deliver terrifying firepower out of its 12 guns in 3x quadruple turret configuration as main armament, and with never-seen-before propulsion capabilities.


However, the Normandie class was cancelled at the outbreak of the first World War, and the 12 naval guns already built for it were transferred to the French army as surplus.
As WW1 progressed, the warring nations switched their production from naval vessels to the more pressing needs of the Western front.

The high demand for super heavy artillery, combined with the sudden availability of surplus naval guns of various calibres saw many of these mounted on carriages, and converted to railroad guns for use on land operations.

The French Army launched a conversion programme for Normandie‘s twelve cannons. Half of them were converted by Schneider to Canon de 340 modèle 1912 à glissement , a railroad gun that transferred the natural recoil of the gun to the section of track behind it.


These were in fact delivered too late to participate in the war. The other 6 guns were converted by St.Chamond (the actual manufacturer of Normandie‘s guns) to Canon de 340 modèle 1912 à berceau, using a hydro-pneumatic cradle system to absorb the recoil.

All of the “cradle” guns saw action between 1916 and 1918, with two of them assigned to the US Army and one reinforcing the Italian army. At the end of the war, the railroad guns were mothballed in reserve.
The outbreak of World War 2 saw the railroad guns entering French service for a second time, and placed along its eastern border with Germany.

After the Fall of France, most of these railroad guns were captured and pressed into the service of the German army. A number of the six “cradle” guns (some sources suggest three) were given to the Italians for evaluation toward the defence of their key Mediterranean naval base at Taranto.

However, the Italians are not known to have actually deployed the guns, for lack of suitable ammunition : the 340mm caliber was specific to the French navy, which may explain this scarcity. And so the last of the “cradle” railroad guns were assigned to the Plouharnel battery in the Atlantic coast.

34 cm cap cepet 5

The battery layout at Plouharnel shows emplacements for 4 railroad guns, not 3. That suggests that maybe the Italians received less than 3 “cradle” guns after all, or it may suggest that not all railroad guns at Plouharnel were à berceau.

Some other sources suggest that St. Chamond only ever converted 4 guns to à berceau in the end, and that all of them were based at Plouharnel. Whatever the truth is, the fact remains that the battery seems to have accommodated 4 railroad guns near the coast.

The railway track branches out between the villages of Plouharnel and Quiberon, allowing the gun carriages to move into the site and rest inside the concrete emplacements.


Behind these, a series of personnel and ammunition bunkers, as well as ancillary structures such as a water cistern and electricity generators. And topping it all, a formidable observation and fire control tower that is still standing today.

Such an important site would be expected to comprise a network of land and air defences too. I saw a type FL242 air defence bunker that would have contained a light or medium gun. Such emplacements are known to have existed in numerous locations along the Atlantic Wall, in places as disparate as Fjell in Norway, the Hague in the Netherlands, or the island of Jersey in the English channel.


They were equipped with a searchlight, ammo stores and space to accommodate a crew of 8 or 9. A 2cm Flak 30/38/Flakvierling would have been an AA gun commonly emplaced in a FL242.
Nearby, artillery and anti-tank defences can be seen. It has been suggested that this might have been an emplacement for 7.5cm PaK 40 anti tank gun, by virtue of a sign on one of its ammo compartments.

I am not certain about that usage, however. Since these strong points seemed to point towards the sea, this might have defeated their supposed utility as AT emplacements.


Plouharnel is a sizeable site, covering 2 sq km. There was much more to be explored than what I could cram into my quick visit. Speaking of quick visits, Feldmarschall Erwin Rommel toured the defences of the Atlantic Wall as the General Inspector of the Atlantic Defences.

He is known to have visited Plouharnel battery in April 1944.
And whatever happened to Batterie Plouharnel?

Well after the Normandy landings, the Allies attempted to assault the heavily fortified Lorient Submarine Base, which by that time had resisted heavy aerial bombing unscathed.

They were unsuccessful : its garrison lasted to the bitter end, finally surrendering on May the 10th 1945.

The well defended Plouharnel battery lasted until March 1945, when it was captured after a prolonged artillery bombardment.

Both sites were used by the French army after the war, and today they are open to the public, a highly recommended visit in case you find yourselves in Bretagne.


I have heard that the some of the guns of Plouharnel (minus the carriages) can still be seen rusting away from the sea in a nearby French army depot, although I haven’t seen these with my own eyes.
Today, the era of Super Dreadnoughts and railroad guns is long gone, as modern navy and artillery doctrines are based on mobility and precision.

But the story of the French 340mm naval guns remains an amazing wartime tale from the time when brute firepower was a key component of naval and land operations.

Resources and further reading:

  1. Axis History Forum
  7. Battleship Lorraine
  8. Operation Dragoon
  9. Battleship Bretagne
  10. 340mm/45 Modèle 1912 gun
  11. Battleship Normandie
  12. Battleship Provence
  13. Operation Catapult