Sabot,

300gr Bloodlines in a black CR according to my calipers squeezed tightly measure .506. My gun loads the .458 Socoms in yellow CR pretty good for 3 shots and they measure .5055ish tightly compressed. I usually shoot 3 of the Socoms then dry patch to make loading easier.

Its hard to get a true measurement. I held the top of the bullet and bottom of sabot and squeezed together then measured around the bottom knurl.