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Yes it stacks, but only on the first damage roll (chain reaction results ignore any modifiers for the following "extra" roll). And don't forget that shooting for separate units is resolved in sequence, so each shot only counts the tanks that shot earlier, you don't get to fire everything, add up the total bonus, and apply it to every hit. And obviously you have to get a penetrating hit, which is far from automatic against the AV 14 most superheavy targets have.
So if your three Baneblades got lucky and caused three penetrating hits you would have a damage roll at D6+2, a roll at D6+3, and a roll at D6+4. And any "chain reaction" results would cause an additional roll with a plain D6.
(And of course this is a significant nerf from 5th edition, where "chain reaction" bonus rolls still got relevant bonuses and a sufficiently large formation of Baneblades would automatically kill any target they scored a glancing or penetrating hit on since they would cause "chain reaction" on a 1+.)
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