I sat down with three Shadowrun books, the main book from the second, third, and fourth editions, and compared their skill lists. You'd be amazed at how different they were. At the same time, I was reading Fudge, which I found to be very interesting in light of the fact I'm writing my own system. For those not familiar with it, Fudge is a game system that is highly adaptable, allowing you to customize it to your tastes. As such, portions of it almost read like an RPG design guide than a rulebook.
On the matter of skills, Fudge discusses the depth vs breadth consideration of skill lists. You can go broad, painting abilities in wide strokes that leave you with fewer skills, or you can go more detailed, meaning each skill covers less ground and characters have more to choose from.
This being Shadowrun, I of course immediately thought of guns. In that context, we're talking about having a single guns skill or pistols, rifles, shotguns, submachine guns, etc. Shadowrun itself has done it both ways over its various editions.
This being a game I'm designing for my particular player group, I decided to ask them directly. One player said he preferred simple rules he could learn and then push aside. Another, a gun enthusiast, said the granularity was important, not only for realism but for the feel of the game. If the game was a game of specialists, you should specialize your skills.
So I'm borrowing a page from the second edition of Shadowrun and I'm compromising. Knacks will be broad (one guns skill), but you can specialize if you like. In fact, there are two levels of specialization, just like there were in SR2. You can concentrate, which focuses your knack, and you can specialize, which narrows it further.
For example, Firearms is the basic knack. You can concentrate in one type of firearm, like pistols. Now your knack only applies to pistols. You can narrow this further by specializing in one specific kind of pistol, such as the Savalette Guardian (because does anyone ever use a different pistol since Fields of Fire came out?).
This gives granularity to those who want it, but provides a basic knack list to those who don't care about the options additional complexity offers and just want it simple.