Will belly drum, stack with weakness policy? Or vice versa however its said?

Question

Will weakness policy increase attack even more if belly drum has already maxed attack? Or would weakness policy be a wast of an item?

Answer 1

No, Belly Drum will still make the Attack Boost automatically to +4 regardless of a Weakness Policy boost. Similar to using Belly Drum on a Pokemon with minus stages of Attack, it will always round up the boosts to +6.

Source: Experience.

Comments

+6, or x4

Answer 2

>The modifiers conferred by moves operate on a sliding scale of stages. When a given stat is raised or lowered, its current stage is increased or decreased by the amount dictated by the move, up to a maximum of +6 or a minimum of -6.

--Bulbapedia

Thus, regardless of Weakness Policy or not, it will gain +6 (x4) Attack.

Tested it on Showdown as well. Weakness Policy + Belly Drum won't work.