Aspect of the Falcon

[Phast]

If you already have the feat that doubles the crit range,
making it 17-20 already...

Does the Aspect of the Falcon double that range?
Making the crit range 13-20?

Or would it give a anything at all besides X3 Damage?

[Mysterious Stranger]

The spell grants specific things so that is all you get.

Second things that increase your crit range never stack. If the feat you are looking at is improved critical the feat itself states that it does not stack with any other effect that expands the threat range.

[Wonderstell]

Aspect of the Falcon wrote:

You take on an aspect of a falcon. Your eyes become wide and raptor-like, and you grow feathers on the sides of your head. You gain a +3 competence bonus on Perception checks, a +1 competence bonus on ranged attacks, and the critical multiplier for your bows and crossbows becomes 19-20/x3.

This effect does not stack with any other effect that expands the threat range of a weapon, such as the Improved Critical feat or a keen weapon.

Generally speaking, Abilities that increases threat range doesn't stack.

[Phast]

Thank you