Britain
12-pdr [3"/50 (7.62 cm)] 18cwt QF Mark I
Updated 28 December 2008
Anti-torpedo boat gun of the late 1800s.  Some of these were converted to AA guns during World War I.  None appear to have survived long enough to serve during World War II.
WNBR_12-45_mk10_Dreadnought_pic.jpg

HMS Dreadnought about 1906-1907
12-pdr 18cwt QF Mark I guns may be seen atop the forward and rear turrets.  These were given light shields shortly before the start of World War I
U.S. Naval Historical Center Photograph # NH 63367
WNBR_75-50_mk2_Shannon_pic.jpg

7.5" (19 cm) turrets for HMS Shannon under construction with 12-pdr 18cwt guns on top
Library of Congress Photograph LC-DIG-ggbain-19618
Click on this picture for a larger image
This image is also at The Vickers Photographic Archive as 6486
Images at Australian War Memorial

Picture of 12-pdr 18cwt on HMAS Swan
Search via "P04447.002"

Picture of 12-pdr 18cwt as AA gun on barge
Search via "H18888"
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Gun Characteristics
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Designation 12-pdr [3"/50 (7.62 cm)] 18cwt QF Mark I
Ship Class Used On Capital ships of the late 1800s and early 1900s including HMS Dreadnought

Used on many World War I Monitors
Date Of Design N/A
Date In Service N/A
Gun Weight 0.9 tons (1.0 mt)
Gun Length oa 154.7 in (3.929 m)
Bore Length about 150 in (3.810 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire 15 rounds per minute
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Ammunition
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Type Separate
Weight of Complete Round N/A
Projectile Types and Weights HE 2crh - 12.5 lbs. (5.67 kg)
Bursting Charge N/A
Projectile Length N/A
Propellant Charge 2.75 lbs. (1.25 kg) MD
Muzzle Velocity 2,600 fps (792 mps)
Working Pressure N/A
Approximate Barrel Life 1,200 rounds
Ammunition stowage per gun Monitors:  300 rounds
Others:  N/A
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Range
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Elevation With 12.5 lbs. (5.67 kg) HE Shell
Range @ 20 degrees 9,300 yards (8,500 m)
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Mount / Turret Data
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Designation PIV
Weight  1.71 tons (1.74 mt)
Elevation -10 / +20 degrees
Elevation Rate Manually operated, only
Train 360 degrees
Train Rate Manually operated, only
Gun recoil N/A
Note:  Some guns were later converted for HA use.  From the Australian War Memorial picture H18888 noted above, these appeared to have used very simple, extemporized mountings.
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Data from
"Big Gun Monitors:  The History of the Design, Construction and Operation of the Royal Navy's Monitors" by Ian Buxton
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Special help from Rod Butcher