I'm not sure about .STL files being available, but I'll be making the raw Halo: Reach turret OBJ file available at some point within the next few days.
That being said, personally I'd advise against assembling this model as a 3D print - it's far too large and heavy, and printing the file wouldn't be cost-effective. My suggestion would be to 3D-print the intricate pieces (barrel, bolts, etc) and then hand-fabricate the rest.