Yeah, unless this Sukuna has massive durability or precog or immortality, a shot in the heart should kill him and he'll shot first (he's basically Han solo' shot first personified).
It did kill Atalante alter (albeit she's hesitating) and will kill Kadoc even before he's activating sirius light (but Anastasia shields him).
To put it into perspective:
A Bolt Action Rifle modified by Magecraft is enough to kill Demonic Beasts if they are shot in the head(To put this into further perspective, a Yaga who is head and shoulders above Humans in every way, requires multiple full force strikes to kill one with a butt end of a gun) can't put down a single Oprinchki unless it's done by multiple shots(like a good dozen or so is required). Billy can atleast kill them in 3 shots BEFORE his 3rd Ascension(and full power) and can mow them down in one bullet... each.
Those Oprinchki that Billy faced was FAR stronger than the ones the Yaga using those Rifles had issues with even before that.
Not to mention even Arjuna had to block Billy's attack back in America meaning it would have hurt him too and we've seen Arjuna take bullshit before and in the Manga so yeah...