The “bullet” appears to be either a hollow die formed sabot like thing or a solid turned from GM. I really don’t know what it is. When I shake mine I “seem” to able to feel powder inside but I cannot say for certain. If there is powder, that would preclude it’s use as a function-test dummy.
Below is a photo of some of the others.
Left is the XM110 with the typical green sabot. HS is WCC 63
Next is the XM645, the second generation with the same green sabot. HS is IVI 71. This cartridge came with at least two other sabots, the aluminized and the cream colored one.
Third is the final iteration (I think) of the XM645 with a clear plastic sabot and a 4.32mm bullet. HS is IVI 69. This was a last attempt at saving the SPIW project.
Next is the XM144. First generation. HS WCC 63.
Finally is the XM216 second generation. HS RA 65. This case is a little larger than the XM144.
As far as I know, AAI was involved in the project from the beginning. I’ve never read that the project was SECRET.
Jim, the second photo attempts to show the “bullet” on the XM144WE4. May not be too clear.
CSAEOD has shown some other flechette cartridges in his posts. Maybe he’ll chime in here.