I've read they mounted the 2A42 on the side, near the centre of gravity to compensate for the recoil of the BMP-2 gun.
From all the sources I've seen, the 2A42 is the same calibre (30mm), but longer barrel and muzzle velocity than most other Attack helicopters
Couple of years later the Mi-28 comes out using the same cannon, but chin mounted, with larger arc it can shoot in. From the limited videos I've seen, I can't visually tell that the Havoc has worse recoil or flight characteristics than the Ka-50/Ka-52 when firing, or that it's a reduced version of the 2A42.
I have no Idea on actual accuracy differences between the platforms.
*If* all the above is correct, was the mounting for the Mi-28 just better designed (more time spent I guess), or did the Black Shark get by then with am economically cheaper and lighter mounting by doing the side mount and hoping coaxial rudder speed would compensate?