There has long been a requirement for a personal defence weapon, or PDW, for soldiers whose primary duty does not involve carrying a rifle. They need something much smaller, lighter and handier which does not burden them or distract them from their main task. Originally, the main priority was for officers, who were almost invariably given revolvers or pistols; but handguns also found a place with cavalry and, later on, with soldiers such as gunners who might come under infantry attack or with tank crews whose space was at a minimum.
The Second World War saw extensive use of submachine guns chambered for pistol ammunition in both offensive and defensive roles, although they were not primarily intended as PDWs. While shorter than a rifle, they were often almost as heavy and troublesome to carry. A commendable effort to find a better solution was made by the US Army, resulting in the adoption of the light and handy .30 M1 Carbine chambered for a much smaller and less powerful cartridge than the M1 Garand rifle. This was so successful that it was often used in more offensive roles than originally intended, especially in its selective-fire M2 version, leading to some criticisms of the effectiveness of its ammunition. However, neither SMGs nor the Carbine replaced pistols, which remained in service alongside them.
A major change came with the adoption of small arms chambered for small-caliber high-velocity ammunition, the first and by far the most common cartridge being the 5.56×45. Its adoption by the USA in the 1960s, and by the rest of NATO in the 1980s, largely killed off the military use of both the .30 Carbine and most SMGs. Rifles and especially carbines in 5.56mm were lighter and handier than traditional SMGs while being a lot more effective. They even replaced pistols to a great extent in many armies, although the US Army has remained an exception.
While the standard NATO pistol/SMG cartridge has always been the 9×19, also known as the Parabellum or Luger, early experiments were also made with small-caliber compact cartridges. Among the many tried from the 1950s to the 1970s, three are particularly notable: the .22 APG, Colt .22 Scamp, and Colt .221 IMP.
The .22 APG (Aberdeen Proving Ground) was an M2 Carbine rechambered for a new 5.56×33 cartridge. This round was not a necked-down .30 Carbine but was based on a wider case, and it fired a 41-grain bullet at around 3,100 fps. This was only for comparative test purposes as a part of Project SALVO in the 1950s, but the other two were serious attempts at producing different kinds of personal defence weapons at the end of the 1960s.
The Colt .22 Scamp (Small Caliber Machine Pistol) was chambered for a 5.56×29 cartridge which fired a 40-grain bullet at 2,100 fps. The gun was basically a big pistol, gas-piston operated and with a plastic receiver to minimize weight, and was marketed, without success, as a .45 M1911 replacement. It could fire semi-auto or three-shot bursts at a cyclic rate of 1,500 rpm.
The Colt .221 IMP (Individual Multi-Purpose weapon) was initially chambered for an existing commercial cartridge, the .221 Remington Fireball designed for the big, bolt-action XP-100 pistol, and fired a 52-grain bullet at 2,500 fps. The ultimate version was intended to use a .17 cartridge firing a 25-grain bullet at 3,000 fps. It was developed at the request of the USAF who wanted a survival weapon, and was subsequently designated GUU-4/P. The gun had a most unusual layout; it was a bullpup without provision for a forward handgrip, the pistol grip being located close to the muzzle. The shooter was expected to steady the weapon by holding the receiver against his forearm with his non-firing hand, which led to the nickname “arm gun”. Like the Scamp, this made no progress although the layout was adopted for the 5.56mm Bushmaster Armpistol.
During the early 1990s, NATO became concerned that potential enemies were starting to issue body armor to their troops, which the 9mm ball rounds were unable to penetrate. As a result, a competition was arranged for a replacement for the 9mm which would have to penetrate a specified level of body armor (named the CRISAT target for the Collaborative Research Into Small Arms Technology project), defined as a 1.6mm titanium plate and 20 layers of Kevlar, while retaining sufficient energy to incapacitate the man wearing the armor, out to a range of 150 metres. Two different weapons were envisaged for this ammunition; a short-range (50 m) PDW weighing less than 1 kg (effectively a pistol) and a medium-range (150 m) close defence weapon weighing less than 3 kg (a compact SMG).
The penetration requirement forced the adoption of a small-caliber cartridge firing a high-velocity steel-cored bullet. The first contender was FN’s 5.7×28 round, as chambered in the P90 SMG and subsequently the Five-seveN pistol. The standard SS190 ball uses a 31-grain bullet fired at 2,350 fps (from the P90). The ammunition is lighter and smaller than the 9×19, allowing the pistol magazine to hold 20 rounds, and the P90 to carry 50. Recoil is also lighter than either the 9mm or the 5.56mm, making the weapons easier to shoot accurately.
The 5.7mm FN was subsequently challenged by Heckler & Koch who introduced their 4.6×30 cartridge at the end of the 1990s, initially available in the MP7 machine pistol (with the P46 pistol intended to follow). The current standard ball loading is the Ultimate Combat, which fires a 31-grain bullet at 2,360 fps (heavier than the original 26-grain Combat Steel). Various trials of the two cartridges were held between 2000 and 2003 and, while both met the requirements, the FN round generally came out ahead. However, the necessary consensus between NATO countries proved impossible to achieve so no standardisation has taken place: it is left to each country to make its own choice.
The Chinese apparently followed the same line of thinking as they introduced a new 5.8mm round for pistols and SMGs. This has the same calibre as their new rifle/MG cartridge, but the case is only 21mm long instead of 42mm, and is more slender. Performance is modest, but the relatively heavy bullet provides good penetration.
Other similar cartridges have emerged in recent years, so far without commercial success, such as the British .224 BOZ (10mm Auto necked-down, offered in a Glock 20 pistol), the Czech .17 Libra (based on a rimless version of the .22 Hornet case and chambered in a conventional SMG-type PDW), the Swiss Tuma MTE .224 VA (based on the 7.62×25 Tokarev case and offered in a machine pistol), the .225 JAWS (developed by Wildey for the Viper pistol on behalf of Jordanian Armaments and Weapon Supplies, with the same case diameter as the .45 ACP; .250, .300, .350 and .400 versions were also made) and, last but not least, the Swedish 6.5×25 CBJ, of which more later.
To date, the 5.7mm P90 and 4.6mm MP7 have achieved only modest military sales, mainly to specialised units rather than for use as general-issue PDWs. This means that the most common combination of self-defence weapons in Western service remains a short-barrelled carbine in 5.56mm (or equivalent) caliber with a self-loading pistol. However, there are problems with both of these weapons when used as PDWs which have been highlighted by the conflicts in Iraq and Afghanistan. These theatres lack front lines behind which soldiers can feel safe; attacks can come anywhere at any time. This means that all soldiers, including transport drivers and others who would not normally expect to be in a combat zone, need to be able to defend themselves.