Japanese 12.7mm explosive bullet used in HO103 aircraft machinegun?
No pic of the mouth? I doesn't appear to have lands and grooves so it might've been pulled and deactivated??
Knowns- CH head stamp is Fabrique, formerly FN, in German occupied Belgium.
Unknowns- probably a 50 cal exploding bullet made for Japan.
I dunno. Unless it was paper patched ala Shooter, it would leave significant lands and grooves unless the rifling was worn out and accuracy would suffer. If I can find my old Weapons of the World from 1950, I will look it up.