The Lewis gun Mk.I had a rof of 550 rpm, increased to the higher rate of around 700 rpm with the introduction of the Mk.II and Mk.III in May 1918.
The Vickers and the Maxim (Spandau) had a reliable rof of around 450 rpm when used with fabric belts in the air. The earliest mechanical synchronisation gears used by both the Germans (Fokker and Albatros types) and the British and French reduced this to around 350 rpm. The introduction of an improved Fokker type with the Albatros D.III or D.V (not quite sure of the exact date) then raised this back up to 450 rpm, but the introduction of a muzzle booster seems to have only improved the reliability of the Maxim and not increased the rate of fire. The limiting factor here appears to have been the continued reliance on fabric belts that would become stiff when cold and wet, and would not reliably support a higher rate of fire. The Germans were the first to introduce the metal disintegrating link belt to replace the canvas belt, but they do not appear to have had access to the right quantity or type of hard alloy to provide a stiff enough belt that would work reliably in the air, and after initial experimental use in the Fokker E.III they went back to using canvas belts.
The British introduced the Constantinesco synchronising gear in March 1917, a big improvement on earlier gears as it did not rely on a mechanical linkage and did not therefore limit the rate of fire by engine speed. This appears to have raised the rof of the Vickers to around 450-500 rpm, still limited by the use of a canvas belt. The first metal disintegrating links appear to have been the Sangster links introduced for land use in January 1916, but not in great numbers, and the Myers Mk.I or "MS" belt for air use in July 1916. These early links, however, suffered from the same problem with the metal alloy as the German type and were therefore rejected for air use, with Col. Sefton Brancker reporting to Trenchard that "the articulated belt is made of too soft a metal and the clips are subsequently jamming". It was only with the Prideaux link first developed in December 1916 that a hard enough alloy was used, and not until October 1917 with the Mk.III version that a good enough design for reliable air use meant that a higher rate of fire could be supported. The introduction of the Hazelton muzzle booster in May 1917 then raised the Vickers rof to 1000 rpm in land use, and it was the combination of the Constantinesco synch. gear and Prideaux links already in use that meant this could then be exploited to raise the Vickers rof to 850 rpm for air use (reduced from 1000 rpm to reduce jams and stoppages) in October 1917. This also appears to have been the time that the ammunition load for the synchrionised twin-Vickers used on the Sopwith Camel went from 250 rounds per gun (2 x 250 round canvas belts) to 500 rounds per gun (2 x 500 round Prideaux link belts), just in time for a change of role for the Camel to ground strafing and attack support.
So, it seems like Rise of Flight introduced the boosted Vickers a little early. Nothing I can do about that. I'm also having a hard time reconciling what he says about the early Spandau rate of fire on the Albatros D.II and Eindecker with Boelcke's 440rpm.
Also, regarding Spandau improvements, Dave Watts measured 440rpm from the gear box of a D.IIIau engine, so the Germans still used that setting in 1918.
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