This came from the Berk Lewis estate. Nothing found in HWS volume 2 or 3, although there is a somewhat similar projectile for a .50 caliber in Volume II, page 241, figure 225D, but it uses a broad copper rotating band with the pre-engraved grooves with clipped corners at the front.
Headstamp F A 42 although the A has been cut away by what is probably an index or alignment notch in the rim.
Bullet diameter .527 over the high points and .488 ahead of the “rotating band” area and in the grooves. Bullet is steel and attracts a magnet.
Neck length reduced so overall case length is 3.60”/91mm instead of the nominal 12.7 x 99mm standard case. Other dimensions appear standard.
Overall cartridge length 5.40”/137mm
Primer in place, does not feel like there is any powder. Bullet is very tight fit, not crimped.
What is this?