(Parts A) "A set of weapon parts. A scope and a slide. A weapon cannot be made from these parts alone."
(Parts B) "A set of weapon parts. A frame and a magazine. A weapon cannot be made from these parts alone."
By defeating Nemesis in the harder modes of Resident Evil 3, you will get items off him. The first lots of items will sometimes contain weapons parts, which will make the Eagle.
Once you have both the Eagle parts and have equipped them to together you will have this handgun, and basically that's just saying you will have yourself a beastly piece of equipment.
The Eagle has many abilities that will help you, there is only one let down. With the Eagle you can't use the special Black Taurus type of ammo, only the original kind. Still regardless of this, the Eagle is a great gun to have on you. I have no idea why they decided to give it the ugly purple hand grip for though.