- You must have a Smartlink vision enhancement either implanted in your natural eyes or as part of cybereyes and some method of connecting them to your smartgun through a DNI in order to receive the full +2 dice pool bonus.
- Using external equipment such as goggles or glasses with the Smartlink vision enhancement grants the +1 bonus only.
- Characters with no Smartlink-equipped gear/augments cannot receive a dice pool bonus from this feature and do not get increased accuracy with their smartgun i.e. the smartgun system is useless without a corresponding smartlink.
Rules as Written
This question arose from the somewhat ambiguous wording of the smartgun system section, from a misconception on my part where I somehow thought it specified that the +2 bonus was for having a DNI (thanks to Archaos for correcting me on that front in his answer); and from the often-interchanged use of Smartlink and Smartgun when referring to the part of the system installed in/on the weapon itself.
While Archaos' answer made a critical correction and David's answer provided an opportunity to make me think about what would happen to the balance of Essence costs if alternatives to using the smartlink vision enhancement were available I still felt like the question hadn't been completely and concretely answered.
My initial confusion most likely came from these two sentences of rules text:
SR5 Core Rulebook, Page 433:
The smartgun features are accessed either by universal access port cable to an imaging device (like glasses, goggles or a datajack for someone with cybereyes) or by a wireless connection working in concert with direct neural interface.
Wireless: A wireless smartlink provides a dice pool bonus to all attacks with the weapon: +1 if you're using gear with a smartlink or +2 if you're using an augmentation for which you paid essence.
On their own these two snippets of rules text seem to conflict in that the wireless bonus section specifies that a smartlink must be present in the piece of gear you're connected to while the earlier text makes no such specification. This left me to confused as to what exactly was required and thus posing my question.
However after reviewing the answers provided here and doing some more reading myself to confirm my suspicions, I came across these two snippets of rules text.
SR5 Core Rulebook, Page 433:
Smartgun system: This connects a firearm or a projectile weapon directly to the user's smartlink.
SR5 Core Rulebook, Page 444:
Smartlink: [...] Without a smartlink a smartgun system just sends out data that isn't received by anyone and has no effect. A smartlink installed in a natural eye or in a pair of cybereyes is more effective than a smartlink installed in an external device.
Looking at it this leaves me in no doubt that a smartlink system must also be present in order for the user to receive any bonus from the smartgun system and that being able to perceive AR (through a DNI or otherwise) has no bearing on the effectiveness of a smartgun system.
Refererring back to the first two sections of rules text this means that to receive the full +2 bonus the smartlink system must be installed in the user's eyes (cyber or natural) and they must connect their smartgun directly to their smartlink either by connecting a cable to their datajack or by a wireless connection to a device which has a Direct Neural Interface such as an implanted commlink.