I am going to agree with ASCorley sort of. I am going to assume assume that this is some sort of home brew or small enterprise modification of a standard 50 caliber tracer projectile. I dont know how you aquire projectiles that haven’t had the tracer pellet pressed in, but scraping out the tracer compound isn’t particularly problematic. There is a guy here in town that makes dummies and those 50 caliber and 20mm ball point pens, he burns out the tracers from pulled down projectiles with an acetylene torch. He has a steel plate that must have about a hundred holes in it that he uses to hold the projectiles base up. It must be quite a show when he lights them off!
To make an easy job of modifying the projectiles it would be nice to have one of those Corbin swaging presses.( corbins.com/ ) Corbin sells polyethylene balls for the purpose of filling space in a jacket, usually in the nose for added stability.
I am not sure why someone would fill the rear of a projectile with plastic other than to make an unstable short range round. Now if for some reason the maker of this round wanted to increase the weight or stablity of the projectile, lead shot could be added and compressed into an almost solid mass with a swaging press.
This is all conjecture on my part. Thinking up stuff like this is what some guys live for and some of those guys may even work for the Army. The first big clue would be to determine if this is a reload. Has the primer crimp been reamed off or swaged out? Is the grey appearance of the interior case walls due to burned powder residue?
Chilled lead shot is very hard (for lead) because it is high in antimony. It is also coated to prevent corrosion that would lead to clumping, which gives shot a darker appearance than plain lead bullets. You could check the weight of the recovered pellets against the weight of of standard shot pellets to see if that is their origin. You could also try to determine if the plastic is all one piece, or made up of compressed balls as the Corbin method would yield.
All of this is just guess work and doesn’t ID the maker unless someone here has more detailled knowledge of experimenters and experimental designs. With all of the projectile forming gear that is available to a guy with a bit of extra cash this projectile could be produced in somebodies garage or in an Ordinance lab. Have fun tracking it down.