Before the errata, some people thought you add the extra AP from APDS modification then multiply. The errata made the writing more explicit to clarify that you don't.
Personally, in my games, I've altered when this extra called shot can be used. I did it to control how people use it, for both in-game sense as well as power control. I ruled that the Bull's Eye Double-Tap/Burst can only be used in conjunction with the Double-Tap, Aimed Burst, or Brain Blaster attacks.
The idea there is that those attack types are the ones that aim at lining up bullets to hit targets more (rather than the default bursts that just send more bullets down-range to make it harder to dodge). So a) it just makes more sense, and b) it keeps some of the cheese down by preventing those high-powered Sniper Rifles using Semi-Auto bursts to get -22 AP...
Really, I always found it odd that this uses weapon AP at all. If the idea is that the bullets are carving the way for each-other, wouldn't it make more sense that the AP from the bullet would be multiplied?