The 75/55-mm tapered bore 7.5-cm Pak 41.
Artillery is not a field in which you might have expected, in 1939, to find anything secret; it appeared to be a fairly pedestrian field of activity, with developments limited simply to making minor improvements in metallurgy or fire control or detail design. Surely there was nothing that one nation could come up with which would have escaped the attention of every other artillery-producing nation? Or was there? The taper-bore guns In 1903 a German called Karl Puff patented a design for a gun in which the bore, instead of being the same diameter from end to end, was tapered; it started out at the breech end at, let us say, 10mm diameter and then gradually got smaller until at the muzzle it would be 7mm diameter. He completed the idea by designing a bullet with an expanded sleeve around its waist; this was of 10mm diameter, so that it loaded correctly into the bore, but when fired it passed down the bore and the gradual taper squeezed the sleeve down until it left the muzzle with the sleeve firmly reduced into a prepared groove in the bullet so that the bullet had the usual smooth exterior shape.
His object in designing this device can be explained by some simple arithmetic. Suppose the base area of a bullet when loaded to be 10 square centimetres. And suppose the gas pressure generated by the propellant charge to be 10,000kg. The pressure on the bullet will therefore be 1,000kg per square centimetre, which will produce some specific velocity. But as the bullet goes down the bore, the base diameter shrinks and thus the base area shrinks with it. The design of the charge can be adjusted to provide a constant gas pressure, so that by the time the bullet reaches the muzzle the pressure remains the same at 10,000kg but the base area has, let us say, halved. So the pressure on the bullet is now 2,000kg per square centimetre, and this will have increased the velocity by a considerable amount. Tapering the bore therefore develops a far higher muzzle velocity than could be achieved by a conventionally rifled parallel bore.
Puff’s intention was to obtain high velocity so as to get a flatter trajectory and a shorter time of flight, and thus improve the accuracy of the weapon. And everybody thought it was rather a clever wheeze, but how do you drill a tapering hole and then rifle it? And how do you make these complicated little contracting bullets? Ah, said Herr Puff, that’s your problem. But if you do succeed, than I’ll take my percentage in licence fees.
Puff’s patent duly expired without ever being worked, and some time in the 1920s another German, a gunmaker this time, decided that technology had moved along a little since 1903 and perhaps Puff’s idea might be workable. The gunmaker was Hermann Gerlich, and in conjunction with a partner called Halbe he eventually produced a taper-bore sporting rifle which he marketed as the Halger. With the aid of the RWS ammunition company he also developed a practical bullet, and, as Puff had predicted, the Halger rifle had high velocity and a flat trajectory which, even though it was expensive, made it popular with hunters, so Halbe and Gerlich were able to make a living. But Gerlich, like many a gun inventor before him, had his eyes firmly fixed on a military contract, and in 1928-33 he walked the corridors of war departments in Germany, Britain and the USA in an attempt to interest them in a powerful sniping rifle. They were all interested, but the cost of such a weapon was daunting, and there were no takers. The Americans experimented at Springfield Arsenal and developed a number of bullets, one of which produced a muzzle velocity of 7,100 feet per second (compared with around 2,800fps for a standard military rifle), but the programme was abandoned early in 1939.
Gerlich went back to Germany in 1933 and got in touch with the RheinmetallBorsig company. Hitler had become Chancellor, the Versailles Treaty was repudiated, re-armament was beginning, and ideas were wanted. They were particularly wanted in the field of anti-tank gun design, because the problem there was to produce a weapon light and handy enough to be easily moved about and emplaced by a couple of infantrymen but powerful enough to penetrate the armour of tanks. Fortunately, at that period, the armour on tanks was not of any great thickness, since it was only intended to keep out ordinary small arms bullets and shell splinters.
Reduced to its basics, the penetration of armour is simply a question of momentum; throw something hard enough and the mass and velocity will carry it through. Even a plain lead ball will go through armour steel if you can get it moving fast enough. So the Halger rifle, with its high velocity, was a promising idea. In order to survive the impact with the target, the bullet was given a core of tungsten carbide; this was enclosed in a soft steel casing which had two flanges or’skirts’which could be squeezed down in the bore so as to present a smooth outline at the muzzle.
Rheinmetall, after various experiments, decided on a barrel tapering from 28mm to 20mm; for security reasons it was known as the schwere Panzerbüchse 41 (`heavy anti-tank rifle’) but in every respect it was a small conventional artillery piece on a two-wheeled, split-trail carriage, with a small gun-shield and a hydro-pneumatic recoil system. The whole equipment weighed only 5051b (229kg) and it fired a 131-gram bullet at 4,595 feet per second (1,400 metres/second) to go through 2.6 inches (66mm) of steel armour at 500 yards range.
It would be idle to suppose that security was so tight that nobody outside Rheinmetall knew what was going on, and there was soon a whisper around the armaments engineers in Europe that somebody was playing with a taper-bore weapon. As a result one or two other people began looking at the idea; one was a Czech who, in 1938, got out of Czechoslovakia just ahead of the occupying Germans and fled to Britain. There he began trying to interest the military in his taper-bore theories. He met with a stony response. In the July 1940 the Ordnance Board, tired of his supplications, poured cold water on the idea.
`The principle has been investigated in the past. It is quite clear that, as regards attack of armour, a weapon of this type cannot be regarded as possessing any advantages over a normal weapon of equal weight and of the same calibre. As regards the application of the principle to the 2-pounder gun, the Board recommend No Further Action.’
A few days later a liaison officer lately returned from France submitted a report on the Halger taper-bore rifle, an example of which he had acquired in France. He got much the same treatment:
`The system was investigated by the Small Arms Committee some years ago (See SAC Mins 1935 or so, under ‘Halger Rifle’), Herr Gerlich himself being employed by the War Department. He was not the inventor of the coned bore and skirted projectile, the credit for this being due to Karl Puff whose patent was taken out in about 1903. [Actually Brit Pat 18601 of 27/8/1904]. The system was developed by Gerlich in collaboration with Halger. It is still being pursued by Kern, in Switzerland, and by Pacetti at Otterup in Denmark. The Kern proposal is being dealt with in current Proceedings. Neither ammunition nor weapons are yet within measurable distance of becoming fit for use in war. No Further Action to be taken.’
About eight months later the British Army captured a Panzerbüchse 41 in the Libyan Desert and flew it back to Britain to be examined. It was found to have a muzzle velocity of 1,388m/sec (4,555ft/sec) and to penetrate 70mm of homogenous armour at 100 yards range.
By that time Rheinmetall had moved on and had designed the 4.2cm Panzerjagerkanone 41 (`tank-hunting cannon’) which was more or less an enlarged version of the first weapon. This started out at 42mm calibre and ejected the projectile at 29.4mm calibre. The shot weighed 11.8 ounces (336g), had a velocity of 4,150ft/sec (1,265m/sec) and could go through 3.43 inches (87mm) of armour at 500 yards range, or 2.36 inches (60mm) at 1,000 yards. This weapon came into service early in 1941.
Krupp, that other famous German gunmaker, had also looked at the taper-bore idea. In 1939, looking well ahead to the inevitable increase in the size and strength of tanks, the army had asked Krupp and Rheinmetall for a 7.5cm anti-tank gun. Rheinmetall produced a conventional gun. Krupp was attracted to the taper bore but was faced with a major engineering problem in producing a tapering gun barrel of that calibre and size and therefore invented a variation which became known as the `Squeeze-bore’.
The 7.5cm Pak 41 gun was a conventionally rifled gun with a barrel 116 inches (2.95m) long. To the end of this barrel was attached a smooth-bore extension 37 inches (950mm) long which had a varying internal taper. As the flanged shot left the muzzle of the rifled section and passed into the extension, it first went through a section tapered at 1-in-20 for about 10.6 inches (270mm), then into a more sharply tapered section at 1-in-12 for another 6.7 inches (170mm) and then into a parallelsided section for the rest of its travel, emerging squeezed down to 5.5cm calibre. The advantage of this method of manufacture was that only a short length of the barrel had to be tapered and this did not have to be rifled. The wear on this taper as the shot passed through at high speed was such that the extension piece was worn out after about 500 shots, but it was simply held on the barrel by a screwed collar and could be replaced in a very short time in the field and with the minimum of tools.
The gun fired a tungsten-cored shot weighing 5.71b (2.6kg) at a muzzle velocity of 3,690ft/sec (1,125m/sec) and could defeat 7 inches (177mm) of armour at 1,000 metres range, striking at a 30 degree angle, or 4.9 inches (124mm) at 2,000 metres. This was a really formidable performance for 1941 and but for one thing this might have been the standard German army antitank gun for the remainder of the war. The one thing which defeated it, and also defeated the other two taper-bore guns, was the demand for tungsten to provide the cores for the projectiles. Tungsten was not native to Germany and had to be imported; the supply was restricted, and there was a constant demand for it for the manufacture of machine tools and other vital production equipment. A tungsten machine tool could be sharpened or rebuilt when worn; a tungsten projectile fired at an enemy was so much tungsten lost for ever. And since production was the more vital of the two conflicting demands, tungsten for ammunition was cut off in the late summer of 1942. And once their special ammunition was gone, the taper-bore guns went to the scrap pile. So effectively, indeed, that few specimens of the 7.5cm Pak 41 survived the war.
But if the shortage of tungsten ruled out the taper-bore as an anti-tank weapon, it certainly did not rule it out in other applications, and now the anti-aircraft specialists began to look at the system. In the anti-aircraft business the principal problem was the interval between firing the gun and having the shell arrive in the vicinity of the target; a great deal could happen during that time, and any way of shortening the shell’s time of flight by increasing its velocity was carefully scrutinised. Therefore the taper bore, with its substantial increase in velocity, was a highly attractive idea; the difficulty lay in the design of the projectile. In an anti-tank gun the `payload’ was a lump of inert metal, but in an anti-aircraft gun the payload had to be high explosive. And the dangers which lay in squeezing a high explosive shell were self-evident. With armour-piercing shot the core acted, as it were, as an anvil, while the tapering barrel acted as a hammer, but with an explosive filling the squeezing action had to be carefully controlled so as not to place excessive pressure on the shell body. Two solutions appeared to work satisfactorily. In the first type, the shell was of smaller diameter than the bore and was fitted with two supporting bands of sintered iron, one at the shoulder and one at the base. These were attached in the manner of driving bands, and performed the same function in spinning the shell, but they were malleable so that as the bore reduced they were swaged down and folded back, so that at the muzzle the shell left with two smoothed-down bands which set up minimal air drag. The second method was rather more complex. Three soft studs were fitted at the shoulder of the shell and the base was deeply indented with a semi-circular groove around the body. Into this grove went a malleable skirt with a circular base which fitted into the groove and acted as a sort of flexible ball-joint, turning backwards as the skirt was squeezed down by the reducing bore. At the same time the soft studs at the shoulder were pressed down and deformed until they were mere bumps on the outside of the shell. Again, the result was that as the shell left the muzzle the studs and skirt had been reduced to streamlined excrescenses which set up minimal drag. It was claimed that either of these designs could give a reduction in the time of flight by about 30 per cent, though there appear to be no trials results to back this up. Like so many other developments, the taper-bore anti-aircraft gun was overtaken by events and the war ended before the design could be perfected.