[JaM]

Multicharge warheads are more effective against chobham armor too.Not only against ERA. They are capable to penetrate different layers of armor more effectivelly than solid HEAT round. (but still they will not be able to penetrate front turret armor...)
About that BK-27, i think that this is just the typo (very frequent issue with russian markings in western media) as 3BK-31 is s projectile for 3VBK-27 round, as 3BK-29 is for 3VBK-25. So BK-31,(not BK-27) is newer than BK-29.