Just about to start the long road of upgrades for on-foot weapons and I'm searching for the best / latest info before sinking time into the process.
Besides what Steve already said, don't worry so much on several weapons. It's okay to stick with 2 or 3 that you like.
Finding pre-engineered weapons is your best bet, since it'll save you not only the few resources for up to the first two levels, but also an engineering, which is the most important, since it means less rare materials in most cases AND unlocking an engineer. And since missions barely require gunfight most of the time, you'll be more pressed to learn the ropes than actually upgrading weapons to do them (weapon levels WILL still be critical in combat zones).
I myself use a TK Aphelion + Karma AR-50 combo (Dominator suit allows two primaries), and the switching is really fast. I end up using mostly the Karma (since most unaltered enemies leave shields down, except some assassination targets), and it's great for even middle/long range (the Karma AR-50 came with Stability, and aim keeps basically dead center while shooting).
Keep in mind that for infiltration/stealth work you need both Audio Masking AND Noise Suppressor, so you're left with two slots for what you want. And that enemy AI uses vision and sound to find you, so even on CZs where everyone is full alert and has radar, you can still pick enemies off one by one if you're fast enough with weapons that don't make sound (since the rest won't hear gunshots until the target shoots back).
And Scope reduces spread on Shotguns?
That's fishy.
If it was "Stability reduces spread" I would understand, but, looking closer reduces spread feels like something that will be patched later. I wonder if changing FOV also changes spread.