The Rolls-Royce Machine Gun was an experimental design by Rolls-Royce during the Second World War. Rolls-Royce were, at the time, manufacturing aircraft engines and parts when production of the M2 Browning (which was fitted to the aircraft that Rolls-Royce were helping to build) hit problems, prompting Rolls-Royce to develop their own heavy machine gun.
The Rolls-Royce Machine Gun was designed to be significantly lighter than the M2 Browning, and this was largely achieved by removing 5in from the barrel. The barrel itself was cut with six grooves (in a right hand twist pattern) to form the Machine Gun's rifling. The barrel was bored for the .50 BMG round, as used by the M2 Browning, on the basis that this ammunition was in use by the RAF.
The Machine Gun's receiver and other parts were manufactured from hiduminium alloy (an unusual aluminium alloy) which further reduced weight. Otherwise the Rolls-Royce operated much like other machine guns of the era. The Rolls-Royce Machine Gun used a recoil operated action with a locking flap system (like the Russian Degtyarev machine gun). The barrel returned to its firing position via a spring while the bolt used hydraulic pressure (caused by the recoil) to return to the firing position.
The Rolls-Royce Machine Gun was designed around the .50 BMG cartridge which (as mentioned earlier) was used in the R.A.F.'s aircraft. The Machine Gun itself used a 250 round belt magazine with a cyclic rate of 1,000 rounds per minute and could produce muzzle velocities in excess of 2,430ft/s (712m/s).
The designers of the Machine Gun, after a few months of development, then decided to use the .55in (13.9mm) calibre bullet designed for the Boys Anti-Tank Rifle. The intention was to make the design more potent than the M2 Browning was.
The Rolls-Royce Machine Gun was never put into production, despite a year of development that received the approval and support of the Air Ministry (whom supplied 2,000 rounds for testing at a time when production and resources were under pressure to produce enough rounds for the British forces). Despite a promising, if destructive test in March 1941, Rolls-Royce decided to use the .55 calibre round of the Boys Anti-Tank Rifle.
The project itself looked to be drawing to a successful close when production of the M2 Browning began to pick up again in 1942. It was this that prompted Rolls-Royce (whom only began the project as the M2 Browning's production was struggling) to drop their experimental heavy machine gun concept in March of the same year, with only one complete example manufactured.
The Illustrated Encyclopedia of Firearms - Ian V. Hogg (ISBN: 0906286417)
Small Arms of the 20th Century - Ian V. Hogg/John S. Weeks (ISBN: 0873418247)