I was wondering if say a bronze arms tribe was powered up by a barkwhip thats tapped. So he's at 3000 atk, can he be crimson hammered??

No, he cannot be Crimson Hammered.

Oh alright thanks, because my brother said, in the rule book, they count the base atk power of the card, so it could be crimson hammered, i thought that was weird

Bronze Arm is powered up to 3000 as long as the Barkwhip that is pumping Bronze Arm's power remains tapped. This means Bronze Arm is 3000 when it's tapped and untapped just as long as Barkwhip is tapped. As long as Bronze Arm has that pump effect, it cannot be destroyed by Crimson Hammer.

The rule book/online FAQ actually says to start by taking a creature's base power and then add to it any effects that have changed that power before targeting that creature for spells like Tornado Flame.

yep, cant be hammered.

ya the bronze arm will be 3000 and unable to be hammered as long as the barkwhip is tapped on the field. The +2000 power will always be with bronze arm as long barkwhip is tapped on the field.

No, it can't be hammered, which is one of the reasons why Barkwhip is one of the best evo's out. 5000, only cost 2!!! Powers up other Beast folks, what more could you want?

Your brother is probably confusing "+ XXXX power" with "Power Attacker + XXXX." When attacking a tapped Power Attacker creature - or using a spell against it, you DO look at it's base power. This is a different situation, as others have already explained.