I made a 1911 cake before:
The gun itself was made from three thicknesses of foamcore covered with fondant. If it's for this weekend you don't have time to do that.
If you're trying to make the entire cake the gun, print a silhouette of the gun as big as you want the cake to be, (here's one I found by googling it http://juggalo-grob.deviantart.com/art/Desert-Eagle-Wallpaper-111660457
) cut it out, trace it on foamcore and cut a board, and then use a sheet cake to cut the cake out of. I would use foamcore for the trigger guard, and cover it with fondant...be sure to tell them that part is not edible...or you could use rice krispie treat covered with chocolate (to make it smooth) and then fondant). You probably need to do the beavertail and the hammer that way too.
One of the most distinctive things about the Desert Eagle is the triangular shape. This link has a picture that shows it from an angle where you can see the shape: http://www.remtek.com/arms/imi/desert/50/index.htm