Maybe it has something to do with Russian markings.
I found this on Vasily Fofanov page:
http://armor.kiev.ua/fofanov/
# 3VBK-7 (3BK-12 projectile) (DOI 1962)
420mm RHA at 0°; Steel liner; A-IX-1 charge (flegmatized RDX); I-238 detonator
3BK-12M modernization utilizes improved 3V-15 detonator (DOI 1968)
# 3VBK-10 (3BK-14 projectile) (DOI 1968)
450mm RHA at 0°; Steel liner; octol charge (flegmatized 75% HMX/25% TNT); 3V-15 detonator
# 3VBK-16 (3BK-18 projectile) (DOI est.1975)
500mm RHA at 0°; Copper liner; octol charge; 3V-15 detonator
3BK-18M modernization introduces wave-shaper boosting penetration by 10% (DOI est.1978)
550mm RHA at 0°; Copper liner; octol charge; 3V-15 detonator
# 3VBK-17 (3BK-21 projectile) (DOI est.1980)
Complex of measures to improve jet formation reliability
550mm RHA at 0°; Copper liner; octol charge; 3V-15 detonator
3BK-21B model (DOI est.1982)
520mm RHA at 0°; DU liner for improved performance on advanced armors and enhanced behind-armor effects.
# 3VBK-21 (3BK-25 projectile) (est.DOI 1985)
Unknown performance and round configuration
# 3VBK-25 (3BK-29 projectile) (est.DOI 1988)
New explosive pressing technology improves jet precision
Weight reduced to 18,400g; MV boosted to 915m/s
Est.650mm RHA at 0°; Unknown liner; tandem charge
3BK-29M presented by NIMI in 1998; unknown improvements over 3BK-29
# 3VBK-25I (3BK-29I projectile) (est.DOI 1988)
Practice HEAT round that matches 3BK-29's flatter trajectory
#
3BK-31?
3BK-31? round
3VBK-27? (3BK-31? projectile) (in development?)
800mm RHA at 0°
680mm RHA at 0° behind external ERA
660mm RHA at 0° behind integral ERA
Unknown liner; triple charge
So maybe BK-27 is 3BK31 round (3VBK-27)