I'll try to word this to make sense.  On the Krinks and SBR AK's with welded on or pinned boosters to meet 16", does the muzzle booster act in any way like a longer barrel?  As in, does it give any higher velocities or result in a more complete burn?  Or is it just taking up space?  The reason I ask is that I am thinking about doing a AK-105 type carbine, and am concerned with having just a 12" barrel with a .223.  With a 4" booster to make it 16", would it still be shooting at 12" velocities?  Thanks.

Muzzle brake I think you mean.  To my knowlege a 12" barrel will gain nothing in velocity or accuracy through the use of a muzzle attachment.  The brakes are intended to tame recoil and muzzle rise.  

Once the bullet passes the crown and breaks the seal, that's it.

I'm talking about the Krink type attachments, not 74 style brakes.  I might not be using the right terminology, but I have seen them refered to as boosters, presumably because they have something to do with the gas operation on short barrels?  That's why I'm confused.  If you have a 12" barrel, then a 4" attachment, does the velocity reflect a 12" barrel because that's where the rifling stops, or a 16" because that's where the bullet and gas exit?