Hotchkiss 3-Pounder

Hotchkiss 3-pounder guns were introduced in 1886 for use in contexts in which the 6-pounder guns were considered too heavy. They were introduced to defend against new, smaller and faster vessels such as torpedo boats and later, submarines. There were several variations produced, ranging from 32 to 50 calibres, although the 40 calibre was the most common. This was the first quick firing artillery to be adopted by the Royal Navy, although it was quickly replaced by the Ordnance QF 3 Pounder Vickers gun midway through Word War One. 

Prior to WWI they were used against torpedo boats, but their small-calibre projectiles were often too light to be overly effective. Following WWI, as other more effective guns were developed, the Hotchkiss was relegated to sub-caliber gunnery training and for use as saluting guns. The gun itself weighs 528 pounds and the mount another 1400 pounds. A rate of fire of 20 rounds per minute was possible, if ammunition was close by and if several loaders were working together, though a more sustained rate of fire was seven rounds per minute. 

Because this secondary purpose as saluting guns prevented the Hotchkiss guns from being discarded, a little under 2,000 of these guns were still available at the outbreak of WWII, which were converted back to shooting guns and were given to small auxiliary crafts and motor launches.

References:
Nav Weaps - Hotchkiss 3-pdr
ASMRB - 3 pdr QF Hotchkiss Gun