So it’s basically a solid copper Nerf football (stabilizing fins and everything) that can be loaded into a standard .223 or 5.56 case.
The purpose is to provide an accurate way to take advantage of a Minesweeper gymnastics level of a loophole regarding their new rifle, which is basically a short barreled rifle without the tax stamp, but with a standard AR style stock as opposed to stabilizing brace. The catch: the rifling is STRAIGHT (hence the need for fin stabilization)
For those unfamiliar with US firearms law, this is similar to a Russian “shotgun” that was able to circumvent laws in Russia regarding semi auto rifles.
More at this link with pictures (posting from mobile phone):