The JM cloche is an element of the Maginot Line. It is a non-retractable non-rotating cupola of steel alloy like GFM cloches, but are armed with twin heavy machine guns, as opposed to the lighter automatic rifles associated with the GFM. There are 179 JM cloches on the Maginot Line. JM is an acronym for Jumelage de Mitrailleuses (twin machine guns). While the MAC 31 heavy machine guns were of the same caliber (7.5mm) as those in GFM cloches, they had a longer practical range 1,200 metres (3,900 ft) and a maximum range of 4,900 metres (16,100 ft), with a 500 round per minute rate of fire.