THE NATO AMMUNITION TRIALS (1977–79)
Although NATO had adopted the 7.62mm NATO round as its standard rifle cartridge, the United States had effectively switched to the 5.56mm M16 during the Vietnam War, and several other NATO countries had followed suit. While it was agreed that the 7.62mm NATO cartridge would be retained for vehicle machine guns and sniping rifles, where its power was essential, a new ammunition trial would be held from April 1977 to select a second, lighter round. This would then become the new NATO standard for rifles and LMGs. Although the NATO trials were intended to select a new cartridge, rather than a new rifle, the British team anticipated that the trials would be a good test of the performance of the new weapon.
Any hopes that the 4.85mm round might be adopted as the new NATO standard round appear naïve in retrospect. Many of the other countries – including the United States – had already made significant investments in adopting 5.56mm weapons, and would have been reluctant to change again unless any new round offered significantly improved performance. Other than Britain, only one country – West Germany – had submitted anything other than a 5.56mm round. The German offering was a very radical caseless 4.7mm round, which did offer very considerable advantages, being only half the weight of an equivalent conventional round and considerably more compact. The technology was not sufficiently mature, however, and it had to be withdrawn from the trials after problems with rounds ‘cooking off’ in weapons after sustained fire. The British 4.85mm round performed better than some of the 5.56mm rounds, but not by enough to justify switching. The best overall performer was the Belgian SS109, a 5.56mm cartridge with a longer, heavier but more aerodynamic bullet than the existing US M193 cartridge, giving it improved range, stability and penetration. The SS109 was therefore adopted as the new NATO standard, though existing 5.56mm rifles could be adapted to use it simply by fitting replacement barrels with the correct rifling twist.
Leaving aside the fact that its 4.85mm cartridge had effectively been rejected, the NATO trials had identified several issues with the XL64 and XL65. The ten XL64 IWs used in the tests fired a total of 66,023 rounds, during which they experienced 679 ‘incidents’. While the XL64 was a new design, and had not benefited from years of troop experience – the American M16 had been in service for more than a decade at that point, and had seen extensive combat use in Vietnam – this ‘Mean Rounds Between Failures’ (MRBF) figure of only 97 rounds was surprisingly poor. The two most common causes of failure, accounting for around 500 of the 679 incidents, were either failures to extract and eject spent cartridge cases properly, or failures with the trigger mechanism and sear. The latter cause of failure manifested itself both as the weapon firing on automatic when set to single-shot, and as the trigger not returning to the normal position after being pulled, thus preventing further firing. In either case, the problem seemed to be caused by grit and dust entering the trigger mechanism – an issue that would recur throughout the weapon’s life.
With the original ‘in service date’ (ISD) of 1983 in danger of slipping, the post-trial appraisal in October 1979 acknowledged that while the basic concept was sound, modifications were needed to improve the reliability of the weapon. It also had to be redesigned to accept the new SS109 5.56mm round and an M16-type magazine, the latter having also been adopted as a NATO standard. Despite the work required, it was decided that User and Ordnance Board trials would begin in mid-1981, to prevent the ISD slipping further. As an interim, ten of the trials weapons (five IWs and five LSWs, all standard right-handed models) were converted to 5.56mm, to allow some testing to proceed while the redesign was in progress.
THE XL70 AND XL73 SERIES (1980–84)
As a result of lessons learned from the NATO ammunition trials, three new weapons (the ‘Production Rifle’ or ‘PR’ series) were produced. These were still chambered for the 4.85mm round as an expedient to get them into production quickly; and they also went through a process of ‘Value Engineering’ (disparagingly referred to by the design team as ‘engineering by accountants’) to simplify manufacture and reduce the time to produce each weapon from ten hours to a more economically viable 7½ hours. This changed the rear body of the weapon significantly, to a wedge-shaped box of pressed metal, despite concerns that this configuration provided less support for the internal components. Other changes included making the flash eliminator a separate component rather than part of the barrel; using a cheaper method to manufacture the trigger mechanism; and adopting a modified gas system and a new (and cheaper) one-piece plastic handguard instead of the two-piece version used on previous weapons. The magazine catch moved to the left side of the weapon, this as a consequence of the way in which the new M16-type magazine locked into place. The safety also reverted to a push-through plunger.
One ‘PR’-series LSW was produced, by converting one of the three ‘PR’ IWs above, and fitted with a new forward-folding bipod. Testing of these weapons indicated that work was needed to resolve the outstanding problems, and it was agreed to delay the User Trials by three months, until October 1981. These ‘PR’ weapons were followed by the main production of the XL70 series, chambered for the 5.56mm cartridge. A total of 51 IWs were built, in both right-handed (XL70E3) and left-handed (XL78E1) configurations. In addition, 34 XL73E2 LSWs were produced, all in right-handed configuration; and since far fewer LSWs would be required, it was decided that no left-handed conversion kits would be produced. Externally, the main difference from the ‘PR’ weapons was a welded-on extension to the magazine housing, which protruded beneath the weapon’s body to give more support to the magazine. Internally, the gas system was modified, the bolt carrier and recoil guide-rod assembly was redesigned, and the extractor was replaced with a new design. Although the LSW had originally been intended to fire from a closed bolt on semi-automatic (for accuracy) and an open bolt on full-automatic (for better cooling), the XL73E2 fired from open bolt in both modes, significantly simplifying the mechanism.
As an experiment, one LSW was modified to fire from the closed bolt on both single-shot and full-automatic, using the trigger mechanism from an IW, and this ultimately became the preferred option. While firing from a closed bolt improved LSW accuracy, the problem of chamber overheating and the attendant ‘cooking-off’ problem – the reason the open-bolt option had been added in the first place – now returned, reducing the LSW’s sustained-fire capability.