Machine guns fire ordinary small arms ammunition carried in loops on a canvass belt (weighing 1 kg. when empty), which is generally capable of holding 250 rounds of 8 mm. cartridges weighing 8.315 kg. These belts are very carefully manufactured, the material being shrunk, so as to prevent their shrinking when in use. Two men can refill an empty belt with 8 mm. cartridges in seven minutes, according to Swedish experiments, while a belt filling machine can refill one in a still shorter time. The rate of fire of machine guns is approximately 500 rounds per minute. They are variously mounted,[278] according to circumstances, on sleds (in which the gun rests on a frame similar to that of a wheelbarrow), on tripods, or, for mountain warfare, on basket frames. Although the greatest readiness for firing was obtained with guns mounted on cavalry carriages (two-wheeled carts, similar to limbers, and equipped with shafts), which also permitted the greatest amount of ammunition to be carried along, these guns offered such a high target that their use, in an infantry action, was entirely out of the question, leading only to their being quickly silenced. Another defect was that the guns were unable to follow immediately upon the heels of the organization to which they were attached. Guns mounted on light tripods possess the least readiness for firing, as the gun must be dismounted during each change of position; but tripods are indispensable in mountain warfare. In India a tripod frame on wheels is employed. The basket frame, which is very light and is carried on the back of a soldier, affords nothing but a rest for the gun. This type of mount has the disadvantage of necessitating, in reality, freehand firing when the barrel is supported near the muzzle; that the operator, when firing continuously, becomes greatly fatigued, and that the accuracy suffers in consequence thereof. The advantages of the tripod and the wheeled carriage have been skillfully combined in the carriage adopted in Germany. In this the gun rests on a sled; this is in turn supported by the carriage proper, which is wheeled. In exceptional cases the gun may be fired from the carriage, but ordinarily it is fired from the sled, which is detached from the carriage for that purpose. This sled permits the gun to be laid at any desired height and enables it to follow the infantry anywhere during an action.[279] [278] British cavalry machine gun with mount 152.3 kg. Machine gun with tripod } 16.5 + 18 = 34.5 „ Machine gun with basket frame } latest model 35.0 „ Machine gun with sled } 16.5 + 24 = 40.5 „ [279] The following complement per gun is considered necessary: Germany 14¹⁄₂ men 9 horses. Switzerland 8¹⁄₂ men 12 horses.