4.5mm FN Experimental

In Buttweiler Auction Volume XI Number 1, he had a 4.5 F.N. Experimental based on a .22 Winchester Magnum impressed “C” headstamp, that had been necked down to 4.5mm (.177). He said it was reported to be a test design used in the development of the centerfire P90 Weapons System. Does any have any information on this test series? What other variations of this rimfire experimental exist (headstamp, neck angles, bullets etc.)?



Here’s a photo with description of some early 4.5 and 22 winmag experimentals from the FN 5.7 project:

The photo of cartridges is from Paul Smith, and he talked about them in this old post: