The McGlashan pneumatic machine-gun was designed not as an offensive weapon, but as a cost-saving training tool for aircrew gunners. It fired BB rounds, small steel spheres about 0.17 inch in diameter. Since the rounds were made of steel they could be recovered from the ground with a magnet. Due to the relatively small and light projectile, range was limited to about 25 metres.
The propelling force was provided by compressed air at between 150 and 400 psi, and possibly higher. The gun air release valve and the feed mechanism were operated by a solenoid powered from a 12 Volt 30 Amp-hour accumulator. The cyclic rate was 300 to 500 rounds per minute, depending on the air pressure used.
The inset picture at top left shows four lead-shot rounds that were fired at a steel plate using air at 300 psi. The round in the centre is unfired.
There is a Wikipedia page, but it gives little information. According to that page the McGlashan gun was used 'in thousands' during the war. Another source gives 2600 guns as an estimate.
Aha! This website gives a great deal of information about the McGlashan gun, and even offers a repair service!
Source: Air Guns and Air Pistols by L. Wesley, pub Cassell, 6th edition 1971