How things stack
Some interesting stacking descriptions
Info status: Published | Jan. 5, 2020, 3:28 p.m.
Movement Speed: There's a MS cap in the game, and the base character speed + speed demon mutation are reaching this cap. Power Armors reduce the base character MS, speed demon is pretty much mandatory for everyone, hence, the movement-speed related perk cards or legendary effects only have any value when used in Power Armor.
Weight reduction: The cap on how much weight can be reduced is 90%. This means that full set of weapon weight reduction armor, 5 pieces each reducing by 20%, will reduce only 90% of total weight.
NPC Trading: The maximum charisma for barter purposes is 25, and there's no benefit afterwards. Hard Bargain perk increases your charisma for barter purposes by 3, up to 9 at 3*.
Explosive negation: Experiments performed by me to determine if Fireproof gives any advantage when combined with Dense armor mod have shown both that it helps and it doesn't, depending on when I tested it. It seems to be one of those things that are fluid during the patching.
AP cost negation: stacks directly with cap at 100% - you can negate all the VATS AP cost of shooting some weapons.