MG151 and 30mm and WGr. It was essentially an A-8 weapons platform, only with 7mm cowl guns. No reason for the A-6, because there's no incentive to fly the A-8. As-is the weapons are the only advantage the 8 has over the 5. Put in the 6 (with identical performance to the 5) and the A-8 never gets used again. Period.
It's better to have the -5 than the -6, because there's some difference between the versions.