10 July 1939. Newcastle-upon-Tyne. England.
The gun division of Vickers-Armstrong weren’t happy with what they’d been asked to do. There was so much work on, that looking at yet another adaptation of a gun was a pain. The Royal Navy had been looking to get as many of their old 12-pdr guns refurbished as possible for use on various second line ships. The 3-inch/45 20cwt QF HA gun had been designed before the Great War and there were 553 Mark I, 186 Mark II, 27 Mark III and 111 Mark IV in stock, and Vickers were working with the Royal Navy to get them fit for action.
The request from the tank division of the company was to look at the possibility of using this gun as the basis for a new tank gun was an unwelcome addition to the gun design teams’ work load. They knew that Woolwich was working on making a 3-inch howitzer to replace the QF 3.7-inch tank mortar used in the Close Support tanks, but that was a different proposition to what was being requested from Sir John Carden.
He wanted a dual-purpose tank gun that would have enough muzzle velocity to beat 3.5 inches (90mm) of armour with an anti-tank round, and an HE shell with a reasonable bursting charge. The team which had been given the task looked at various solutions, from the easy to overly-complex. But, with all the other work that was being done, just about every element of the company’s capacity was backed up already. Since the gun was a pre-Great War design, to bring it up to date, would mean that the design team would almost have to begin almost from scratch. A few shortcuts might be available, someone suggested using the new 25-pdr’s breech and recoil system and marry it to the 3-inch barrel. The problem was that the army was screaming out for as many 25-pounder guns as they could get their hands on. Any complications to the order for artillery guns would be frowned upon.
The only machinery which was standing relatively idle was the line for the Model 1931 75mm AA gun. This had been a particularly good seller: Romania and Denmark had bought a license to build it themselves. Belgium, the Netherlands, Lithuania, Finland, Turkey, Switzerland and China had all bought numbers of guns directly from Vickers. It hadn’t been taken up by any of the British armed forces, 75mm wasn’t a calibre it used much. Finland’s order had been rechambered to take their 76.2mm x 505R, so there was the equipment to make it a 3-inch gun if required, which would simplify sourcing ammunition.
The problem with transforming a gun designed as an anti-aircraft weapon and make it into a tank gun wasn’t insurmountable, but it wouldn’t be easy. First of all, the weight of the gun, just under three tons, would have to be stripped down as far as possible. The barrel length at 10 feet and seven inches, and the recoil length would either need to be reduced, otherwise the turret would look more at home on a ship than on a tank. To achieve this, and keep the muzzle velocity between 2000 and 2500 feet per second, to give it the penetration required, was all a difficult juggling act.
With the drawings of the Close Support Valiant turret to work with, the team had used the 3.7-inch howitzer’s fittings as the basis for attempting the forced marriage. It would take some doing, but they were able to send a report saying that this was the most feasible answer to the request. The draughtsmen’s opinion was that with the current size of the turret it would probably mean that there wouldn’t be room for three crewmen. If they were to progress to a working model, it would be necessary to get the go-ahead from the Board of Directors, to provide the funds to actually build the gun and then fit it to a turret to check for balance and all the other things that went into getting a tank gun working properly.