Armament Research Services (ARES) is a specialist technical intelligence consultancy, offering expertise and analysis to a range of government and non-government entities in the arms and munitions field. For detailed photos of the guns in this video, don't miss the ARES companion blog post:
With the British military's return to Afghanistan in the Global War on Terror, it was found that the very long engagement ranges made it necessary to have a 7.62mm designated marksman's rifle, in addition to the 5.56mm scoped rifles in service. This was not unique to the British military; the US came to the same conclusion and addressed it by reissuing M14 rifles in new stock systems. The British, however, put out a tender for a new 7.62mm rifle in 2009 (the FAL being not sufficiently accurate to meet this need).
The final contenders for the role were the H&K 417 and the Lewis Machine & Tool LM7 rifle - basically a Stoner SR25 or AR-10 using the original Stoner quasi-direct-impingement action. LMT was chosen as the winner and formally adopted as the L129A1 in October of 2009, with rifles entering field service by May of 2010.
The L129A1 has proven effective and popular enough that the British military is currently looking to expend its role to that of sniper support weapon. This would involve a change in anticipated targets to enemies wearing body armor, which the current 7.62mm projectiles are not particularly good at defeating. A new projectile is in development with a steel penetrator tip (in principle similar to the 5.56mm SS109 projectile) to allow the L129A1 to fill this new role as well.