The HMWSR2-P was a propotype of a light sniper rifle for Spectres operatives under developement on Earth when Reapers invaded. Only a few of these rifles existed and were held on the armory of the central Alliance military comand by the time of the attack.
It was designed as a highly reliable rifle, concieved to operate under very limited conditions where electronics are usually Jammed or used for target detection, like geth territory. For the same reason, most of their sistems are mechanic, and rather archaic for the average eye.
It's unknown how this rifle was found in South America after the reaper war on Earth. Apparently it was taken from the ruins of the alliance command center after the first attack, becoming the weapon of several soldiers while they were retreating from the urban areas, many of them travelling south. The original scope of the rifle was lost in some point of the war.