The most difficult engineering challenge in any rapid firing gun, then as now, is how to supply the firing mechanism with ammunition at the required rate in a way that will not cause jams, and can easily be replenished. This page looks at three of the more successful ammunition feed mechanisms - the Broadwell Drum, the Accles positive feed magazine, and the Bruce feed. There were many patents taken out on alternative mechanisms which are not discussed here.
Hopper Plate for a .65 Naval Gatling Gun (NFC)
Broadwell Drum for .65 Naval Gatling Gun (NFC)
Broadwell Drum for 1874 Gatling Gun
Locking and Unlocking Broadwell Drum (rollover)
The Broadwell Drum was designed by Lewis W Broadwell, and patented in 1870 (U.S. Patent No 110338). The drum consist of two parts - a base plate which engages with slots in the gun's hopper and is thus stationary, and the cartridge assembly which can be rotated about a central spindle to align each column in turn with a cutout in the baseplate.
For rifle calibre ammunition the drum would have, typically, 16 columns each holding 15 cartridges for a total of 240 rounds. The animation of the 1874 Gatling Gun shows a 240 round Broadwell drum, however, photographs in the Gatling Gallery show guns with smaller capacity magazines. Smaller capacity magazines are also used on the Naval .65 calibre Gatlings, presumably because of the weight of a loaded drum.
The .45 calibre Gatling used by the British Army and Royal Navy fired a .45 inch lead bullet of 480 grains, with a charge of 83 - 87 grains of black powder, and used a solid drawn brass cartridge case. The weight of a cartridge was 787 grains, or 1.8 oz. The Broadwell Drum magazine weighed 22 lbs (10 kg kg) empty and 50 lbs (22.8 kg) when loaded with 240 rounds.
The Broadwell patent drawings show drums containing 20 columns of 20 rounds each. While this may have been practical with early ammunition, the higher powered rounds in use by 1874 were longer and heavier. A 400 round drum would have weighed over 70 lbs (30 kg), and would have had a significantly greater diameter than the 240 round magazine adopted by the United States and British forces.
The Broadwell Drum in Action
The Broadwell drum is a very basic ammunition hopper. It is placed on top of the hopper plate with registering strips aligned with groves in the hopper plate. This ensures that the cutout in the baseplate is above the hopper chute. The cartridge assembly is then rotated by hand to align each column of bullets in turn above the hopper mouth. There are no catches or clips to hold the drum in position, and aligning and repositioning the columns is due to the skill and experience of the ammunition number. Each column has a weighted indicator shown how full each column is.
The ammunition number has to pay attention. When one column is emptied, he has a short period of time, roughly equal to 3 discharges, to align the next column to ensure there are no interruptions to the rate of fire. He places his left thumb on the thumb piece (see rollover of unlocked drum) and his left forefinger on one of the lugs on the lower part of the drum, and then brings them smartly into line, at which point the next cartridge column will be aligned over the baseplate cutout, and the cartridges fall into the hopper chute.
The hopper chute itself holds 3-4 cartridges ready to be picked up by the cartridge carrier, and so long as the new column can be aligned before the reserve is taken up, there will be no interruption. Note that if there is a delay, all that happens is that some barrels do not pick up a cartridge.
Charging the Broadwell Drum
To load the Broadwell drum, it is inverted, and the baseplate rotated until the cutout is above a cartridge column. To minimise the risk of cartridge jams, the cartridges are loaded carefully into each column. The indicator is lifted so that each cartridge can be placed in position, and then lowered to allow the next cartridge, and so on until the column is filled.
The drum is then locked by turning the baseplate to align a clip on the right hand registering strip with a lug and clipping it in position. This ensures that the cutout is midway between cartridge columns, so the ammunition cannot fall out.
The Broadwell drum was not best suited to extremes of elevation or depression – since the cartridges simply fall under gravity, and the hopper is simply a chute. At extremes of elevation or depression, it may give rise to many jams as the cartridges would fall further backwards or forwards into the cartridge carrier than normal.
This limitation would be addressed in later magazine designs - to ensure the cartridge is 'positively' positioned and fed onto the cartridge carrier.
There is also the issue of centre of gravity. Note that the drum is held in place by gravity, and positioned on a small central spindle. If the gun were depressed significantly, for example from the fighting top of a warship, there is a real risk of it simply falling off.
Accles Positive Feed Drum
Accles Positive Feed Magazine, Patent No. 290622, dated 18 December 1883.
Test movie showing the mechanism of an Accles drum magazine. The cartridge bases fit into a 'spiral' (actually a series of concentric rings with a step changeover) and are driven along this 'spiral' by a 'propellor'. Lugs on the propellor engage with the cartridge carrier of the gun, so as the gunner turns the firing crank, the cartridge carrier is rotated, and this turns the propellor and drives cartridges directly into the carrier slots. This is a positive action, and cartridges can be fed faster than they would if they simply fell under gravity (as in the Broadwell drum). The gun can also be fired at higher and lower elevations that with previous magazines, since the cartridges are presented in the correct alignment regardless of the gun's elevation.
The Accles drum, however, has the disadvantage that it can only be loaded by feeding cartridges into the exit slot one at a time while turning the propellor. Lucien F. Bruce, who worked for the Gatling Gun Company, invented a mechanical Charger for Accles drums, U.S. Patent No. 341371, dated May 4 1886.