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UPDATE: One correction to make; this rifle has the A2 charging handle. The original A1 version was essentially identical to the standard conventional charging handle. Sorry!
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In 1999, the UK Ministry of Defense put out a tender for a new rifle for UK Special Forces (UKSOF). The elite units of the British military were definitely not going to be using the L85! There was some competition (including the SIG 550 series), but it was pretty much known going in that the contract would be going to Diemaco (later Colt Canada) for a version of their C8 SFW ("Special Forces Weapon"). That was the case, but only after very extensive trials, which actually cost more than the procurement contract itself. The rifles were tested in all environmental extremes, including Alaska, Kuwait, and Brunei.

The rifle ultimately adopted had a number of unique features. It was at heart a Diemaco C8, with Diemaco's early flat top upper (which predates Picatinny adoption, and is actually a bit closer to Weaver - but still compatible with modern accessories). Two barrel lengths were purchased, 10.0 inch and 15.7 inch. Other details include:

Stepped buffer tube
Textured telescoping stock
Permanently attached rubber buttplate
Lone Star grip
Knight's RAS with locking clamps on both top and bottom rails
Strengthened gas block (usually but not always)
SureFire 216-A flash hider
Unique castle nut details
Ambidextrous charging handle

The barrel profile chosen for the L119A1 is quite heavy, and the 10 inch barreled version is substantially overgassed. The guns were heavy, but very reliable, and have since been adopted as the standard service rifle of the Royal Marines. The SOF opted to seek out a replacement around 2013-2016, and that would result in the L119A2 (a significantly different rifle).