Shanks AP is definitely above BM, and other top tiers bar maybe Luffy, Oden & Kaido for now
Since he's swordsman with powerful haki, he has better lethality. He has more chances to KO most guys in 1-3 blows than other top tiers can
That's why he's better in AP than BM
for the record BM attacks with napoleon on kid and law didn't even have Koka/adcoc.
That's why it's silly to compare her attacks with Shanks/Kaido who constantly uses adcoc + ryou on their attacks.
This is why people should stop "Shanks seems a tier above, or way stronger" no he doesn't. He's only using adcoc & ryuo on his attacks more while BM didn't
Also Kid and law had plot armour vs BM yet BM still beat them high diff before they kicked her down extreme difd.
Without plot amour, BM was gonna mid diff Kid 1v1 easiy , upper-mid diff law and kid 1v2.