Where Next For PDWs?
© Anthony G Williams
This is an edited and extended version of an article which first appeared in Jane's Defence Weekly on 15th February 2006
Amended 29 May 2011
With thanks to Max Popenker for permission to use material from his military gun site
The Personal Defence Weapon PDW is a relatively new term for an old class of weapon: the self-defence gun for troops whose duties do not involve carrying rifles. They need something light and compact so that it can be routinely carried (and therefore instantly available) without interfering with their other duties, and also for use by vehicle crews (the interiors of AFVs and attack helicopters being rather cramped). It also needs to be used effectively without much training. It does not need an effective range of more than about 150 metres.
The traditional solution throughout the last century was the handgun, but in reality this has never made a satisfactory PDW. While being light, compact and easy to carry, its practical range in the hands of non-specialists has always been extremely limited, especially in the stress of combat. In fact, handguns can be more of a hazard than an asset, being a common source of NDs (negligent discharges) and allegedly being responsible for more friendly than enemy casualties. Their use increased in the street-fighting of Iraq, especially within the US forces, apparently as a back-up in case of the failure of their rifles, but it could be argued that a more reliable rifle might be a better long-term approach. Rather surprisingly, the British Army acquired around 4,000 SIG SAUER P226 pistols in 2008 and issued them on a large scale in Afghanistan, but after initial enthusiasm their popularity declined and their utility remains the subject of debate. Despite this, it seems that pistols continue to feature in the Army's future equipment plans.
The problem is that handguns require a high degree of training and constant practice in order to achieve and maintain proficiency. This is fine for specialist troops who use them in an offensive role, but much less suitable for a PDW. With admirable foresight, the US Army recognised these problems and introduced the self-loading .30 M1 Carbine in 1942, primarily as a handgun replacement. Chambered for a much smaller and less powerful cartridge than the .30 M1 (Garand) rifle, the Carbine was a light and handy weapon which saw service in more offensive roles than were originally intended, particularly in the M2 version which introduced a selective fire capability.
Other armies seeking to improve effectiveness turned to the sub-machine gun firing pistol ammunition. Introduced in the First World War, they were commonly carried by NCOs in the Second, as well as specialist troops and in the case of the Soviet Army whole regiments of infantry. The support gained from firing from the shoulder gives these a far longer effective range than a pistol; around 100 metres or so. They are also more compact and handier than rifles but not usually any lighter, and in practice have been used more for offensive than self-defence roles. In the last quarter of the 20th Century, SMGs were gradually replaced in many armies by shortened versions of the standard rifles, which in the new small calibres (5.56 mm and 5.45 mm) are just as compact as SMGs, usually lighter, and have the benefit of simplifying training and logistics.
Currently, the most common combination of compact weapons in most armies consists of a short carbine and a handgun. The US Army, for instances, relies primarily on the 5.56 mm M4 Carbine and the 9 mm M9 pistol. Despite this, there is a body of opinion that neither weapon is entirely satisfactory in the PDW role: the pistol for reasons already given, the carbines because they are designed around unnecessarily powerful ammunition (firing 4.0 g bullets at 838 m/s from the M4, for a muzzle energy of 1,400 J). This not only means that the guns are bigger and heavier than a PDW should be, but the increased muzzle flash and blast resulting from their short barrels does not help in training non-infantry troops to shoot accurately (already a problem given the limited small-arms training they usually receive).
Body armour and the NATO response
Another factor which has come into play concerns the development of body armour using modern synthetic materials; a weapon unable to penetrate the more commonly-found levels of armour is likely to prove of increasingly limited value. NATO recognised this problem and in 1990 commenced the task of defining the characteristics of the ammunition required to replace the NATO standard 9 x 19 pistol / SMG round. The key to this was the ability to penetrate a specified level of body armour (named the CRISAT target for the Collaborative Research Into Small Arms Technology project), defined as a 1.6 mm titanium plate and 20 layers of Kevlar, while retaining sufficient energy to incapacitate the man wearing the armour, 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 first contender was FNs 5.7 x 28 mm round, as chambered in the compact P90 SMG and subsequently the Five-seveN pistol. This penetrates the CRISAT target by means of its small calibre and high velocity (a 2.0 g bullet is fired at 715 m/s from the P90, for a muzzle energy of 511 J). The ammunition is lighter and slimmer than the 9 x 19, allowing the pistol magazine to hold 20 rounds, and the P90 to carry 50. Recoil is also lighter than either the 9 mm or the 5.56 mm, making the weapons easier to shoot accurately.
At first, it seemed that NATO standardisation for the FN ammunition would follow without opposition, until Heckler & Koch introduced the 4.6 x 30 cartridge at the end of the 1990s, initially available in the MP7 machine pistol (with the P46 pistol to follow). The performance as currently loaded matches the 5.7 mm (a 2.0 g bullet fired at 720 m/s for 520 J) with the smaller calibre helping with penetration and energy retention. Various trials were held between 2000 and 2003 and, while both rounds met the requirements, the FN cartridge generally came out ahead (the performance of the 4.6 mm has been increased since then). 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.
NATO has recently (2008) decided to revise the CRISAT armour specification and is working on two standards, for soft and hard (ceramic) armour. The implications of this for ammunition performance requirements is not yet clear.
A disadvantage of the small calibre / high velocity route is often argued to be the lack of terminal effectiveness in soft tissue. There is no escaping the fact that the 5.7 mm and 4.6 mm rounds are smaller and less powerful than the 5.56 x 45, which has itself been criticised for lack of effectiveness, and produce smaller wound channels in ballistic testing. However, the true comparator should be the 9 x 19 military ball loading, which the PDW rounds are intended to replace. Proponents of the small calibre rounds point to the fact that unlike conventional pistol ammunition their pointed bullets lose stability and rotate lengthwise on impact, creating a much larger wound channel than the small calibre would suggest. Some anecdotal accounts of the use of the P90 in action support this, while others (especially from US police sources) report a worrying lack of effectiveness. This seems to parallel experience with the 5.56 x 45; small-calibre high-velocity (SCHV) bullets sometimes work spectacularly well but on other occasions fail badly, depending on exactly where the bullet strikes and how it performs thereafter. Large calibre bullets tend to have a more consistent effect.
One attempt to answer this problem is the 6 x 35 cartridge from Knight's Armament Company in the USA. This is made by shortening and slightly necking-out the 5.56 x 45 case to take a 4.2 g bullet which achieves 740 m/s when fired from a 254 mm barrel, developing a muzzle energy of 1,150 J. It is claimed to slightly exceed the muzzle energy of the 5.56 x 45 when both are fired from 254 mm barrels, while producing considerably less flash, blast and recoil, and to be effective out to 300 m. However, it cannot meet the NATO PDW requirement as it is unsuitable for use in pistols: it is purely a carbine round, and is really the spiritual successor to the .30 M1 Carbine cartridge (7.62 x 33). While this might make a useful supplementary weapon to those armies still equipped with full-power 7.62 x 51 rifles, it is hard to imagine that any army using 5.56 x 45 weapons would want to add a calibre which is so close in performance to the 5.56mm.
The ideal would of course be to combine effectiveness against both protected and unprotected targets. Some point out that the West's current opponents do not usually wear body armour, but it would be very short-sighted to assume that this will continue to be the case. Some also observe that no cartridge in the PDW class can hope to penetrate the kind of body armour currently being worn by military forces (at least Western ones) so it is best to forget about AP ability and concentrate on soft target effectiveness, relying on headshots to deal with armoured targets. But there are, of course, various classes of body armour, and insurgents may not choose to wear the very bulky and heavy hard-plate military issue. Furthermore, if well-armoured opponents became the rule this would shift the balance of advantages back to the SCHV route, as the flat trajectory, light recoil and large magazine capacity of 5.7 mm or 4.6 mm weapons would increase the probability of achieving a headshot.
The Russian and Chinese approach
Perhaps the most interesting recent development has been the Russian adoption of the 9 x 19 round, just as NATO was searching for its replacement. In this case, the 9 x 19 is seen as significantly more effective than the long-established 9 x 18 Makarov (an attempt to boost the performance of the Makarov round up to NATO 9 x 19 levels with the 7N16 high-pressure loading was abandoned because of the danger of using this in older guns). The effectiveness of the new Russian 9 x 19 is significantly enhanced by the development of new loadings which combine the ability to pierce body armour with the benefits of a full-calibre bullet against unprotected personnel. It achieves this by means of a method of construction which is similar to that of Second World War APCR, HVAP and PzGr 40 anti-tank gun projectiles: the bullet has a hard sub-calibre core contained within an outer sleeve and separated from it by a polyethylene layer.
If the bullet strikes an unarmoured target, it holds together to produce a wide wound channel. On impact with armour, the sleeve is stripped away and the core penetrates alone. Two different loadings have been introduced: the 7N21 (5.3 g at 460 m/s for 560 J muzzle energy) and the 7N31 (4.2 g at 600 m/s for 756 J - both figures are from pistol barrels). The latter in particular is a very high-pressure loading which can only be used in particularly strong pistols. Not satisfied with improving the 9 x 19 in this way, the Russians have also introduced a longer 9 x 21 round for special forces weapons to achieve the same effect. The 7N29 AP loading fires a 6.7 g bullet at 410 m/s for 560 J, and will reportedly penetrate two 1.2 mm titanium plates plus 30
Russian 9mm composite AP bullet, showing the penetrating core and the remains of the sleeve after hitting armour
layers of Kevlar at 50 m. This Russian approach appears to provide the best of both worlds. The disadvantage is that high impact velocity is needed for this to work, so the bullets (especially in the 9 x 19) are relatively light to maximise the muzzle velocity. This does mean that they will lose velocity relatively quickly, limiting their effective range.
The Chinese have also recently adopted the 9 x 19 round in a DAP-92 steel-cored loading (8 g at 400-430 m/s, for 640-740 J). This appears to be primarily for police use, however: the military are adopting a steel-cored 5.8 x 21 which is a little less powerful than the FN 5.7 or the HK 4.6 mm rounds: it fires a 3.0 g bullet at 530 m/s for a muzzle energy of 420 J (from an SMG barrel). Both of these rounds are used in pistols and SMGs, and are credited with penetrating a 1.3 mm 232 helmet steel plate, plus 50 mm of pine wood block behind it, at up to 100 m range.
The USA - PDW, or a return to the .45?
Mixed signals are emerging from the USA. Experience with the 9 mm M9 pistol in Iraq has led some in the US Army to regard this calibre as unsatisfactory in terminal effectiveness (the military being limited to using full metal jacket rounds rather than the considerably more effective expanding bullets commonly used by police forces). However, the likely direction of any change is hard to fathom. There was considerable interest in 2006 in selecting a new pistol in .45 ACP calibre. US forces have always kept some .45 pistols in service, most recently the H&K Mk 23 selected in the mid-1990s for offensive use by Special Forces. Comparative trials with a variety of pistols chambered in 9 mm, .40 S&W and .45 ACP were carried out, and the USSOCOM (Special Operations Command) subsequently announced that it intended to issue a solicitation to obtain a commercially available non-developmental item (NDI) Joint Combat Pistol (JCP) system, Calibre .45 (ACP). There was to have been an initial order for 24 Engineering Test Units (ETU's), but the maximum quantity expected to be acquired was more than 600,000 pistols: enough to replace the 9 mm pistols altogether. In the event, SOCOM cancelled its plans in September 2006.
Only a couple of months later, the USAF announced its intention to acquire a new pistol, probably in either .40 S&W or (preferably) .45 ACP calibre. This plan too was put on hold in 2007. The US Coast Guard has recently changed to the .40 S&W calibre.
The US has of course had a strong emotional attachment to the .45 ACP calibre since its adoption almost a century ago, but this new interest was surprising as the cartridge has little ability to penetrate even the most basic body armour. And in any case, the difficulties non-specialist troops are likely to experience in hitting targets with a pistol in combat have already been referred to.
In contrast, the USMC did acquire a dozen each of the P90 and MP7 in the early 2000s for evaluation, but no results ever emerged. However, it was reported in March 2007 that the US Army is looking to acquire a new PDW. The head of the Army's "Program Executive Office - Soldier" stated that a formal requirement for a PDW which would be "larger than a pistol and smaller than a carbine" was being drawn up at the Fort Benning infantry centre, and that the Army's small-arms development centre at Picatinny Arsenal would be in charge of overseeing the procurement. So far there are no indications as to the calibre or type of weapon, or whether an off-the-shelf buy or a new development would be favoured. Given the recent track record of several recent US attempts to acquire new small arms, some scepticism as to whether this will lead to anything seems to be in order: indeed, nothing has been heard of this since.
PDW cartridges, shown to scale. From left to right: .30 Carbine, 5.56mm NATO, 6x35 KAC, 5.7mm FN, 4.6mm HK,
5.8x21 Chinese, 6.5x25 CBJ, 9x19 NATO, 9x21 Russian AP, .40 S&W, .45 ACP
The guns: what's the choice?
The choice of guns is of course intimately tied to the choice of calibre. One possibility is to retain the current common practice of issuing a 5.56 x 45 carbine (as a matter of interest, the M4 Carbine is rapidly becoming the US Army's standard infantry weapon, replacing the M16 rifle). In fact, the USMC announced in summer 2007 that in future almost all officers going into combat would be issued with the M4 rather than the M9 pistol. While much more effective than any pistol, the M4 is clearly not an ideal solution: a PDW should be small enough to be carried on the person without it being a burden or interfering with the soldier's primary task, or alternatively be easily stowable in (and deployable from) a vehicle turret or aircraft cockpit. The M4, although fairly light (2.52 kg empty, 3.0 kg with a loaded 30-round magazine), is 838 mm long when ready for use (757 mm with butt collapsed) despite its short, 370 mm barrel.
To some extent, the M4's problems could be addressed by adopting a much more compact 5.56 mm weapon: which means a bullpup. One already in service is the British Army's L22A2, a short-barrelled version of the SA80 intended for AFV and helo crews. This is certainly short at only about 555 mm overall, but it is heavy and in any case is not available for anyone else to purchase: the last SA80s were made some 20 years ago and the L22s were adapted from existing weapons.
The author trying out the L22A2. It was very comfortable to shoot, the weight making the recoil negligible and the forward handgrip being a definite plus. However, I might not have enjoyed it without the ear defenders...The 4x SUSAT is inappropriate for the use this gun is designed for (as well as adding too much weight and bulk), a 1x holographic red dot sight would be much better.
One modern bullpup which is certainly available is the Israeli IWI Tavor. The most compact version is the Tavor X95 (550 mm long with a 330 mm barrel, and 2.98 kg) which is available in either 5.56 x 45 or 9 x 19. Another is the Steyr AUG, available with barrels of 350 mm or 407 mm, with overall lengths of 647 and 704 mm respectively, and weights of around 3.5 kg. However, although quite short, all of these 5.56 mm bullpups are rather bulky.
Earlier in 2007 a private US company, MagPul, announced its PDR (Personal Defence Rifle), a remarkably small bullpup at just 457-508 mm long with a 267-317 mm barrel, and a projected weight of not more than 1.8 kg. Details and illustrations are HERE. This is still in the development phase and it remains to be seen whether it would be tough enough to withstand the rigours of military life, but it looks worth exploring by those armies which have decided on a 5.56 mm PDW (and can bring themselves to consider a bullpup).
The disadvantages of short-barrelled 5.56 x 45 guns in terms of excessive power bringing considerable muzzle flash, blast and recoil, have already been mentioned, and clearly the shorter the barrel and the lighter the gun the worse these will be. Short barrels also significantly reduce the terminal effectiveness of the SS109 / M855 bullet. These considerations led to the development of the KAC PDW in their new calibre of 6 x 35. It weighs 1.8 to 2.3 kg depending on barrel length (20 cm or 25 cm), and with a 20 cm barrel the gun measures 660 mm with stock extended, 440 mm folded. This is a handy and efficient carbine but, as mentioned in the ammunition section above, the cartridge is probably too close in performance to the 5.56 x 45 for an army to want both.
The diagram below shows the calculated recoil energy factors for typical loadings of various PDW cartridges. When fired in guns which are of similar weight and design, the recoil energy developed by the gun will be proportionate to these factors.
The pistol-calibre PDWs
This takes us to the PDWs designed to use the same cartridges as the service pistols, which must therefore be short enough to fit into a handgrip magazine, as well as ballistically suited to very short barrels. Ammunition currently in service in this category is therefore the 9 x 19 (NATO or Russian), 9 x 21 (Russian), 5.7 x 28 FN, 4.6 x 30 HK, and 5.8 x 21 Chinese. Some older weapons in the obsolescent Russian (now Chinese) 7.62 x 25 Tokarev pistol/SMG calibre are also still around. Traditional SMGs are heavy due to their use of a simple blowback mechanism which requires a massive bolt, but the new generation use locked-breech mechanisms, making them much lighter and more compact.
Examples of current weapons are given below. They all have folding or telescoping stocks except for the FN P90 and Chinese Type 05, which are bullpups.
FN P90 calibre 5.7 x 28. Weight 2.54 kg empty (3.0 kg w / 50-round magazine), barrel 263 mm, length 500 mm.
HK MP7 calibre 4.6 x 30. Weight 1.5 kg empty, barrel 180 mm, length 540 mm (340 mm stock telescoped)
SAAB-Bofors CBJ calibre 6.5 x 25. Weight 2.65 kg empty (2.9 kg w / 30 round mag), length 565 mm (363 mm stock telescoped)
Chinese Type 05 Calibre 5.8 x21. Weight 2.2 kg empty, length 500 mm (bullpup), 50-round 4-row magazine
Brugger & Thomet MP9 Calibre 9 x 19. Weight 1.4 kg empty, barrel 130 mm, length 523 mm (303 mm stock folded) 15-30 round mags
Various guns have been developed by the prolific Russian arms industry (these are shown on Max Popenker's site). To pick three examples:
SR-2 Veresk. Calibre 9 x 21. Weight 1.65 kg empty, barrel 172 mm, length 603 mm (367 stock folded), 20-30 round mags
PP2000, Calibre 9 x 19. Weight c.1.4 kg, length 582 mm (340 mm stock folded), 20-30 round mags
AEK-919 Kashtan. Calibre 9 x 18 Makarov. Weight 1.65 kg with empty mag, barrel 167 mm, length 485 mm (325 mm with stock telescoped).
|In 2008 the author recently had the opportunity
to fire several pistol-calibre PDWs, as shown on the right (from top to
bottom): 9mm HK MP5K (folding stock), 9mm Steyr TMP (the precursor to the
B&T MP9), 4.6mm HK MP7 (stock collapsed) and 5.7mm FN P90. Unfortunately,
range restrictions meant that only semi-auto fire could be used, but the
results were still of interest.
The MP5K is the only gun with a traditional SMG layout, but it was the most comfortable to shoot and had no more perceived recoil than the 4.6mm and 5.7mm guns.
The Steyr TMP had no shoulder stock, which drastically reduced its accuracy even on single-shot; it would have been an uncontrollable bullet-spray on auto.
The MP7 was comfortable to shoot, but needed an optical sight.
The P90 was less comfortable than the other stocked guns: the position of the forward handgrip is much less intuitive than the others, and the hands are very close together. The optical sight also has a faint T-shaped aiming mark rather than the brighter red dot which I prefer.
The photos below show the different weapons in the aim: notice the varying elbow angles, and how cramped the P90 position is.
Below left: Steyr TMP. Below right: MP5K
Bottom left: MP7. Bottom right: P90
The B&T MP9 (top) compared with the HK MP7, to scale (both are fitted with Eotech holographic sights).
Note that the MP9's stock folds rather than telescoping, with the butt lying alongside the foregrip. The MP9's foregrip is fixed (although a folding one has been developed) while the MP7's folds backwards.
The Indian Army has been pursuing an interesting track, developing their own 5.56 x 30 ammunition for use in compact carbines. Originally this was available in the MINSAS, a shortened version of the INSAS 5.56x45 rifle, but more recently these has been offered in the more compact MSMC, which features the magazine in the handgrip. This was undergoing Indian Army evaluation in 2010. No illustrations of the ammunition have so far emerged, although the dimensions are 5.56 x 30 and it is apparently just a shortened 5.56 x 45. It is therefore very similar in concept to the 5.56 x 30 Colt MARS of the 1990s, except that the performance as stated so far is very much lower: it is stated to fire a 2.6 g bullet at 650 m/s for a muzzle energy of 550 J, which is little better than the much slimmer 5.7 x 28 and 4.6 x 30 can manage.
An alternative approach to the PDW has been developed by CBJ Tech of Sweden: the 6.5 x 25 CBJ cartridge. The cartridge has the same overall dimensions as the 9 x 19, and 9 mm guns can fire the 6.5 mm round with just a change of barrel (they can use the same magazines). The standard ball round fires a saboted tungsten 4.0 mm diameter sub-projectile at high velocity: 2.0 g at a muzzle velocity varying between 730 and 900 m/s depending on barrel length, with the lower figure from a 120 mm barrel (giving muzzle energies varying between 533 and 810 J), thereby achieving good armour penetration out to a claimed 400 metres from the longest (300 mm) barrel. Other sub-calibre and full-calibre loadings have also been developed.
CBJ also designed their own gun to fire this ammunition, the CBJ MS (a compact SMG with a clip-on bipod) and at the time of the NATO trials this received marketing support from SAAB-Bofors, but this company is no longer involved. The parent company continues to develop the ammunition and is in discussion with possible manufacturers; the focus now is in adapting existing pistols and SMGs rather than pushing the MS.
In mid-2009 the author was contacted by Mr Johansson, the founder of CBJ Tech, who had read an earlier version of this article and been inspired to get together with Brugger & Thomet to produce a version of the MP9 in 6.5mm CBJ. The prototype was being tested and I was invited to Sweden to see it for myself. This combination was still in the development phase (complicated by a recent change in the cartridge case which affected its capacity) so some fine-tuning was still required. Even so, the potential was evident, as I found when I had the opportunity to try the gun in both 9mm and 6.5mm versions, in semi-automatic and automatic fire.
The MP9 is one of those guns which just feels "right" to hold and shoulder. It comes naturally into the aim, and is very comfortable to shoot as well as surprisingly controllable for such a light weapon. Even in 9mm, short bursts could be grouped closely enough to keep the rounds on target out to perhaps 20-25 metres. The 6.5mm version was even better, as recoil felt slightly lighter. The 6.5 mm's high muzzle velocity of around 800 m/s provides a flat trajectory, giving a realistic effective range of about 200 metres in semi-auto fire. I witnessed armour-piercing and gel tests of the CBJ ammunition, the latter being compared with 9mm ball as shown below. I came away with my view of the potential of this gun and ammunition combination very much confirmed. My only complaint was that I couldn't take the little gun home with me...
More information on CBJ ammunition is available from their website HERE.
Top: some views of the 6.5mm MP9 above, fitted with a 1.5x Trijicon ACOG sight (note the cartridge case in mid-air).
Above: three of the CBJ cartridges shown next to a 9 x 19 for comparison: sub-calibre tungsten ball round; full-calibre frangible; full-calibre High Energy Transfer (HET)
6.5mm CBJ results:
Left, penetration of 9mm thick armour plate from a Russian armoured vehicle by the standard saboted tungsten ball round, achieved from a 300mm barrel (900 m/s). The diameter of the hole is about 6mm. The 5.56mm SS109 steel-tipped ball caused a shallow gouge, the 7.62mm lead-cored ball just left a splash mark.
Below, tests in 10% ballistic gel block 340mm wide, shooting direction from left to right:
top - 6.5mm sub-calibre ball after penetrating CRISAT target
middle - 6.5mm full-calibre bullet: the HET and frangible bullets produce the same results
bottom - 9mm ball, for comparison
There is clearly no agreement on any aspect of the choice of PDW; not the type of cartridge, nor the type of weapon. There are no "right answers", just a range of options with different pros and cons. Selection also has to be considered as part of the "suite" of weapons and ammunition available in any particular army. So this author, observing from the sidelines, is sticking his neck out in making any suggestions. However, here goes...what follows is my own armchair (or, at best, shooting bench) opinion.
Pistols do not make effective PDWs; they are simply too inaccurate in all but highly-trained hands, especially in the stress of combat. To achieve even a passable hit probability, a PDW should have a shoulder stock and preferably a fore-grip; a combination which dramatically improves its usefulness over any pistol. An automatic fire option improves the effectiveness of suppressive fire, a useful facility in keeping down the heads of ambushers when escaping or awaiting aid.
For a PDW to be available immediately when required, but out of the way at all other times, it is important that it should be compact enough to be carried attached to a soldier's webbing or harness, if not in a holster, and light enough for it not to become a burden (these will be carried day in, day out, but rarely used). Compactness is also a major virtue when stowing the gun in vehicles or aircraft, and when deploying it from vehicle hatches.
In the interests of reduced weight and controllability, a PDW should use a less powerful cartridge than the military rifle / MG round; it therefore makes sense for it to share ammunition with the service pistol (should there be one).
A PDW is best fitted with a compact, zero magnification, holographic, red dot optical sight.
All of this suggests one of the pistol-calibre PDWs listed above, or something very like them. The bullpup designs have the advantage of being immediately available to fire without having to unfold anything (they are also more comfortable to aim and shoot), but the disadvantage in this role of being appreciably bigger. A folding-stock gun with the magazine in the handgrip is therefore indicated. If there isn't time or space to unfold the stock, the gun can still be used as a pistol.
Of this type of gun, the HK MP7 is currently the military market leader in the West, but many are concerned about the terminal effectiveness of the little 4.6 mm bullet. The optimum choice for them, at least for the time being, would appear to be the Brugger & Thomet MP9 in 9 x 19 (developed from the Steyr TMP). With the Swedish M39/B AP loading the 9 mm is very effective against soft body armour. If more penetration is required, then a loading like the Russian composite AP could be used. Alternatively, it could easily be rebarrelled to take the 6.5 x 25 CBJ round in order to achieve the NATO PDW range and penetration targets, in which respects it is superior to the 5.7 mm and 4.6 mm rounds. It is also a bit lighter and more compact than the MP7.
However, that is just one view: others will have good reasons to reach different conclusions. Far from the future choice of the optimum PDW clarifying, it seems to be becoming more confused. One thing does remain clear: there will be a continuing need for a compact but effective selective fire weapon for a variety of roles, and it is likely to become increasingly important for this to have the potential to penetrate at least some levels of body armour.