I pulled the A2 style hider off of my colt sporter 2 and noticed that there is a "dead space" internally of perhaps an 8th of an inch between the forward end of the threads and the exit hole.  This space had quite a bit of carbon build up so I assume the end of the barrel does not actually abut to  the FH  exit hole, allowing the carbon to build up in this space.  Normal?  if not how to fix?

Thanks for your help.

Yes, normal. Get a Vortex and you won't have this. Not that matters.