British
6-pdr / 7cwt [2.244"/43 (57 mm)] QF Mark IIA
Updated 02 March 2007
The Mark IIA was a modification of the army's Mark II anti-tank gun fitted with Molins auto-loading gear.  Used by many MTBs in power operated mountings to give them a bigger "punch" when attacking barges and small vessels.  It was also proposed to use the Mark II without the auto-loading gear as a wet mount for submarines but this never made it out of the prototype stage.

The Molins auto-loading gear carried seven rounds, one in the breech and six in the feed system.  An additional twelve rounds were carried in ready racks on the mounting.  The last round could not be fired, a precautionary measure to prevent the feed from running out of ammunition.  This weapon allowed semi-automatic firing.

These guns were of monobloc construction and actual bore length was 42.87 calibers.
WNBR_6pounder_m2a_MTB-476_pic.jpg

MTB.476 showing 6-pdr Mark IIA on bow
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Gun Characteristics
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Designation 6-pdr / 7cwt QF Mark IIA
Ship Class Used On MTBs
Date Of Design 1940 (naval version)
Date In Service 1941
Gun Weight 760 lbs. (345 kg)
Gun Length oa 101 in (2.564 m)
Bore Length 96.2 in (2.444 m)
Rifling Length 78.2 in (1.986 m)
Grooves (24) 0.02 in deep x 0.22 in (0.51 mm x 5.59 mm)
Lands 0.0737 in (1.872 mm)
Twist Uniform RH 1 in 30
Chamber Volume 100 in3 (1.693 dm3)
Rate Of Fire 40 rounds per minute cyclic
18 rounds per minute practical
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Ammunition
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Type Fixed
Weight of Complete Round 12.25 lbs. (5.56 kg)
Projectile Types and Weights HE - 6.0 lbs. (2.72 kg)
Bursting Charge N/A
Projectile Length N/A
Complete Round - up to 24.85 in (63.1 cm)
Propellant Charge 1.31 lbs. (0.59 kg) NF 029
Muzzle Velocity 2,150 fps (655 mps)
Working Pressure 20.5 tons/in2 (3,230 kg/cm2)
Approximate Barrel Life 4,000 rounds
Ammunition stowage per gun N/A
Note:  The MV given above is for a Mark IIA gun firing 6 lbs. (2.72 kg) HE shell.  The Mark II anti-tank gun firing 6.28 lbs. (2.85 kg) AP shell had a MV of 2,845 fps (867 mps).  Only HE rounds were carried by MTBs.
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Range
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Elevation With 6.0 lbs. (2.72 kg) HE Shell
Range @ 12 degrees 6,200 yards (5,670 m)
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Mount / Turret Data
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Designation Single Mount
   MTBs (1):  Mark VII
Weight  1.719 tons (1,747 kg)
Elevation -10 / +12 degrees
Elevation Rate Manually operated, only
Train 360 degrees
Train Rate 40 degrees per second
Gun recoil 29 - 30 in (74 - 76 cm)
Note:  This mounting appears to have been adapted from the twin 20 mm Oerlikon Mark V mountings.
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Data from
"Naval Weapons of World War Two" by John Campbell
"Destroyer Weapons of World War 2" by Peter Hodges and Norman Friedman
"Warships of World War II" by H.T. Lenton & J.J. Colledge