Well, I've read through the assorted threads, regarding the topic, re-read the book's connected paragraphs a dozen ties and it still isn't clear to me. I thought it might worth asking, if the community reached a consensus since then.
So, how I see it at this point:
a, Smartgun in the gun, Smartlink either in external device, or in 'ware. Gives the +2 to accuracy and the +1 to attack pool (if run wirelessly). You can do this with wire (goggles, glasses), or wireless (goggles, glasses and contacts).
What makes me think this is that the example gunslinger adept does have contacts (which can't be connected via cable, because, well, contacts...) and has Smartlink on them (with no image link, mind you). Why would he, if it doesn't give any benefit? It didn't get errata'd either.
b, With cybereyes, the same as above, but +2 to attack pool (more direct connection).
c, If, on top of the above, you have some sort of DNI and either wire, or wireless (maybe just with wireless), if it's an external device, you'd get to use the nice free action stuff (ejecting clips, changing fire modes, etc.). Or are they accessible by simply having the smartlinked external device and the smartgun? I don't know, eye movements, AROs, voice?
Am I right? I've seen in the other threads some people stressing the need for DNI for any of the wireless stuff, but then again, why the sample adept has smartlink in contacts, without any kind of trode, or datajack?
There isn't anything official on this in the errata document/thread, or the missions FAQ. I'm doing a search on the "Rules clarifications/FAQ" thread above, but I thought it won't harm to ask.