Charles Manville developed this weapon in the 1930s as a riot control tool, and they were built in 12ga, 25mm, and 37mm. We should point out that the 12ga version was for tear gas rounds only (like today’s 12ga flare launchers) and not safe to use with high-pressure ammunition. Anyway, it was intended for use by prison guards and riot police, offering a much greater ammunition capacity than any other contemporary launcher.
During World War II, Manville tried to sell the military on a high-pressure version to fire 37mm explosive rounds, but was unsuccessful. Instead, the Manville company spent the was making parts for the Oerlikon 20mm AA guns, and the tooling for the gas launcher was all destroyed.
We had the chance to take a closer look at one of the 25mm guns (not 26.5mm or 27mm as sometimes described):
While we were not able to take that one out for a test firing session, we did come across this video of someone else doing so. It looks to be a digitized VHS tape, and we take no responsibility for the “trained professional” pointing it right at his cameraman every time…but it does show a Manville in action:
US Patent 2,101,148 (C. J. Manville, Machine Projector, March 11, 1935)