The L85A2 is described as:
The British military’s standard issue rifle.
You can pick it up in far north Espiritu Santo province, in the north-east corner of the map in the basement of a large house next to a large church. The “La Santera’s Chapel” mission is nearby. The weapons case is in a store room in the basement, next to a row of jail cells.
Attracted by a sniper overlooking the area, I approached this via the jungle to the west which provides some elevation and a great vantage point. If you ignore the church where the story mission is, there are about a dozen enemies in and around the house, and several more pairs patrolling the grounds.
With some time and patience you can take many of the patrols out with a sniper rifle and sync shots before advancing on the house. Make sure you’ve tagged all the enemies first using the Rebel Spotting skill, because there are several levels to the house and they move about between floors and could surprise you. In the house, take enemies down one-by-one with your suppressed pistol.
What can I say about the L85A2? I have no love for this rifle, despite having great range and accuracy stats, which are only beaten by the Mk 17 (found in Flor de Oro). It’s recoil snakes left and right as it goes upwards when firing full auto. It’s also a butt-ugly gun (does that matter?), and there are just better options available from assault rifles in my opinion.
It takes a standard 20 round magazine, and a 30 round extended mag is available in Mojocoyo as well as a 50 round large magazine (you can pick up in Libertad) but I would avoid using this gun personally.
HOWEVER… the challenge of collecting this gun was a lot of fun for me, so maybe it’s worth picking up for that alone.