In the 1960's,
DARPA was contracted to build an anti-tank rifle.
The rifle was a smoothbore chambered for .50 caliber rounds.
The flechette was originally made from depleted uranium, held in place by a sabot, which leaves the round as it exits the barrel.
According to
sources online, the round was propelled from the rifle at an amazing
4500FPS and could penetrate up to
2 inch thick armour plate, and the most amazing part was it could penetrate armour at angles as high as
60 degrees, which would cause most bullets to harmlessly bounce off the armour!
This is as close a replica I could get using rough dimensions available and some logical application of CNC machining techniques.