The Maginot Line - A misunderstood history

Cupola GFM Model 1929, type A

Observation/machinegun cupola

During combat, the duralumin (see this wikipedia article) mounts of the A models proved to be the weakest point. Designers were aware of this, and introduced the B cupola in 1934.

The plan was to modify all existing cupolas by means of field modifications. However, this was a costly process, and the process of upgrading was still in progress when the Germans invaded France.
Cupola type A, modified to B These modified/upgraded cupolas are easily identifiable by the large bulges around the embrasures.
PO L'Einseling has a unique cupola with two types of embrasures.

A wooden platform Inside, all the cupolas had a metal and wooden platform that could be raised or lowered to adjust for the height of the soldier occupying the cupola. In case of injured people, it could be lowered all the way down to the base of the cupola.
Type C cupolas did not have this platform.

The cartridge collection box In the centre of the cupola was a pipe which extended through the platform to the base of the cupola. Spent shell cases fell down the tube into a ventilated bin located at the bottom of the cupola.

When manned by the crew – either one or two soldiers (depending on the model of the cupola) – the cupolas were equipped with observation equipment, weaponry and magazines for the weapons.

The horizontal field of view/fire through the embrasures was 72 degrees. There were six different gun mounts, depending on the role and placement of the cupola.

Gun mounts
Standard gun mount Firing and observation
(elevation-declination: +30% en -30%)
Standard gun mount modified
(placed 10 cm lower)
Firing and observation
(elevation-declination: +50% en -50%)
Special gun mount
(cupolas variants PMA or GM)
Firing
(elevation-declination: +30% en -30%)
For observation
(elevation-declination: +30% en -70%)
Gun mount type A Firing
(elevation-declination: +30% / -64,5%)
Gun mount type B Firing
(elevation-declination: +30% / -70%)
Gun mount type C Firing
(elevation-declination: +61% / -30%)

GFM Model 1929 type A During combat, the greatest nemesis of these armoured cupolas were not the German indirect fire artillery pieces, but the accurate anti-tank guns with their high velocity, armour piercing projectiles. There are plenty of combat reports of AT gun crews firing as close by as 1 km or less, and penetrating the cupolas with ease. Especially the 88mm gun proved 88mm Flak gun to be a cupola and a concrete 'slayer'. At a range greater than 1,8 to 2 km’s and further, accuracy and penetration power were less of a threat to the cupola and it’s occupants.