Dave,
You either have an intermediate development piece used to work the kinks out of the design, or the front end and core of a real round, and a repro tail end that someone made, who doesn't know what it should look like. I'm leaning towards the latter also because it isn't painted, and doesn't look to be te same age as the front end.
The thing needs a boat-tail with a real tracer and a means to hold it in the pot (carrier) till it leaves the bore. Every sub projectile I have ever seen is boat-tailed to hold velocity, which is the most important thing with this design. When it comes to KE rounds, length-to-diameter, penetrator composition, and velocity are the three important factors for penetration and affect. This is a short L/D penetrator, so it needs velocity.