The R5 RGP is described in the game as:
A gun built to defy harsh conditions.
It can be found in northern Monte Puncu province, in a Santa Blanca Agronomic Center. The weapons case is on the top floor of a building in the north of the complex.
Unless there’s a story mission at this location I’m not aware of, there were no enemies here when I approached. It’s perfect for taking a helicopter here, with no resistance and a clear landing area outside the target building, it’s great! Just walk up and take the gun.
The R5 RGP is the assault rifle of choice for Unidad, and basically an upgraded version of the P416 you start the game with, but with a lower rate of fire. It takes all the same accessories, so you get a 20 round standard magazine, with a 30 round extended mag (found in La Cruz) and large 50 round magazine also available in Caimanes, this gun is very capable and a safe bet.
I think there are better ARs like the ACR, but that’s my personal choice. I don’t think you’d go wrong with the R5 RGP.