The M40A5 is described as:
US Marines still use this reliable bolt-action.
You can pick this up in northern Itacua province, outside the door of Itacua Echo rallypoint. The weapons case is on the ground by the low wooden fence.
You should already have this rifle as it’s the first weapon you collect in the game if you follow the missions. It’s right there at the extraction of Amaru at the end of the first mission “Amaru’s Rescue”. This is a rebel rallypoint so there are no enemies here.
The M40A5 is a bolt-action sniper rifle with almost no recoil whatsoever, making it a fantastic introduction to the sniper rifles, and no doubt why it’s the first one you find naturally in the game.
However its shortcomings soon become evident and you’ll want to try some of the other rifles out. Being bolt-action means it has a super slow rate of fire, and it doesn’t accept all the sniper accessories like a Range Finder or buttstocks.
In its favour for a bolt-action rifle though is the standard magazine which is 10 rounds, meaning you don’t really need the 20 round extended magazine (found in Media Luna).