Just to add more confusion, I have a THV bullet whose diameter is .390" . An odd diameter for a rifle or a pistol
It is of recent production but I have no info about the gun that could fire it.
Recently some custom rifles were chambered for .395" cal. wildcats ( .395 Tatanka). A bit larger than the pictured bullet.
Could the base of bullet be about .395 - .408"? The part of the bullet that is outside the case could be slightly smaller than the actual diameter
I measured my .408 CheyTac sample. It is assembled with a .408" monolithic bullet but it is .399" if measured just beyond the case mouth