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The prototype was completed and fired in November 1931, and by the middle of the month it was firing strings of two and three rounds.
Changes to the feed mechanism were all that remained, and by the end of the year it was operating at 130 rounds per minute.
In practice firing rates were closer to 80–100 rpm (1.3–1.7 rounds per second), as the rounds were fed into the breech from four round clips which had to be replaced by hand.
This seemed to be the solution they needed, improving firing rates to an acceptable level, and the work on a prototype commenced soon after.One attempt to solve this problem used zinc shell cases that burned up when fired.This proved to leave heavy zinc deposits in the barrel, and had to be abandoned.Power for the sights was supplied from a 6V battery.British 40mm L/60 includes the British-designed Stiffkey Sight, being operated by the gun layer standing on the right.