This gun is described in the game as:
Most powerful semi-auto handgun in the world.
You can find it in north-west Libertad province, inside FOB Oso Hormiguero – a large Unidad base. The weapons case is inside the armoury to the east edge of the base.
This is another base with a drone jammer, but with enough range and zoom you should be able to fly far enough away to get a view of the base. The easiest approach is to sync shot the snipers in the towers, then enter the base from the north side, and work your way down the interior wall to the east. Unidad presence is fairly low, and you may only have to eliminate one or two soldiers before you get to the armoury.
The D-50 (yes, it is the “Desert Eagle”) is a hand cannon, prioritising damage above all else. You can give this monster a go for some fun, but it’s not the most suitable handgun for stealth kills. The high damage profile should help with armoured targets, however.
There’s very little this gun accommodates by way of attachments. Further letting it down is the magazine, which can only squeeze seven of the 50 cal. rounds in. And there’s no extended mag option either.