In this particular case the shell came first and then it was realised that the principles behind the round could solve an Ordnance Board requirement for a new infantry support gun of extremely light weight.
The shell in question is called a Cambridge Projectile. It was developed sometime in 1943. The idea behind it was for a flame weapon with much longer range than a normal flamethrower. To achieve this it needed to have a high capacity, and of course the highest capacity you can get is a cylinder, so that's exactly what it was, a plain cylinder filled with flammable liquid. If you rotate a cylinder of liquid, like you would a normal shell, it will become very unstable due to centrifugal forces. So it was decided to fire it from a smoothbore weapon. Of course a blunt nosed shell fired from a smoothbore will also be horribly unstable in flight and thus very inaccurate. However it was discovered (possibly by someone at Cambridge, hence the name?) that the if sheath of unstable air was brought behind the centre of gravity the shell would then become very stable.
During development they were aiming for accuracy at extremely long ranges, it was at this point the obvious occurred, if you can achieve a satisfactory degree of accuracy at long range at much closer ranges the accuracy would be extremely good.
This combined with the Cambridge shell's high capacity meant that it had a number of envisioned uses such as Flame, Incendiary, Smoke, Chemical Warfare, Hollow Charge and Low Shrapnel HE. The difference in capacity can be seen from this table:
So with a clear advantage and having overcome the main disadvantage normally associated with a blunt nosed shell the rounds moved onto the testing stage.
It was found that the shells would often deform at the base when firing due to the pressures. So the same ballistic witchcraft that created the sheath of turbulence was applied to the issue of the shell, and the scientists overcame the issues. (To be honest at this point I lost track of what the document was talking about as I'm no ballistics expert!)
Rounds for the 3" mortar were developed and tested. But despite the advantage in payload the amount of material was still insufficient for a suitable flame shell.
In July 1943 trials were carried out with a round for a 3" howitzer, of the same type mounted on the Churchill I and the Matilda CS tanks. Its interesting to note that both these and the later 75mm guns are rifled, yet the very principle of the round needs it to be fired unrotated. However the document I saw doesn't mention how this was dealt with.
As the 3" was obsolete by this stage 22 rounds were manufactured for the Royal Ordnance 75mm. In comparative trials against rotated 75mm shells in May 1944 the Cambridge shell was considerably more accurate up to 2500 yards, despite being lighter and having a higher payload.
|The test gun|
Recoil from this gun was managed by it having a very heavy steel barrel of around 200 pounds, and a pair of small springs to deal with the remainder of the recoil.
|Test round strike on a tank|
Its role was to defeat any enemy tank it would meet and defend infantry. The reason for its lightness was so that it could keep up with the infantry in terrain that would prevent normal AT guns being brought up, such as if the infantry crossed an anti-tank obstacle or penetrated a mine field. Minimum range was to be 500 yards, and at that range the weapon must be able to hit a stationary five foot square target, or a moving five foot high by 15 foot long target doing 10mph. Rate of fire was to be at least five aimed rounds per minute.
The gun built to test fire the Cambridge shell was used as a starting point. The gun was called the "Camal gun" (Or in one entry in the documents the "Cam-Al gun"). This is presumably due to the abbreviation of "Cambridge" which is "Cam", and the light weight portability that resembles Camel guns from the previous centuries.
The heavy steel barrel was replaced by one made of RR77 Aluminium alloy. This has a weight one third that of steel. In fact the majority of the gun was made out of this alloy. This brought the barrel down to a weight of 47 lbs.
The recoil was managed by adding an oil type recuperator slightly modified and taken off a Vickers S gun, and the breech was an interrupted screw type. Even so the recoil was too great to allow it to be fired on a tripod giving 360 degree arc of fire, so they had to settle for the minimum in the requirement which was 90 degrees.
The final version of the mount had a tripod with two long rear legs and a shorter front leg. However the requirement also called for it to be towable. So the gun was designed that the front leg could be removed and replaced with a simple axle and two light car wheels, while the longer rear legs became the towing bar. Once it arrived the axle and wheels could be simply unlatched and the normal leg fitted.
|The final design|
Muzzle velocity of the shell was recorded as 710 feet per second, and with 5 degrees of elevation on the gun barrel the round travelled 818 yards. In addition a HE and WP shell were developed for the gun.
The document I saw doesn't go into the fate of the Camal gun. But I suspect it's likely that as the war was winding up and advances in other man portable AT weapons caused its demise.