18-21 brinel bullets are VERY hard. That makes me think they might be hardened, then tempered, even though with some alloys, like linotype at 21 brinel and monotype at 24 brinel are available, or were at one time.
Those harder "type" alloys tend to be brittle though, while an alloy crimp-on WW bullet can be hardened all the way up to about BH 34. They can then be tempered back down to whatever hardness you wish, through heating and cooling.
A ductile bullet of brinel 18 would to 21 would indeed be fairly non-deforming and give very deep penetration.
The larger the bullet diameter, the softer it can be at higher speeds. Thus, a brinel 18 .45 cal. bullet can be driven faster than a .30 cal. bullet of the same brinel.