Those are all just learned techniques that I don't really teach, but the natural scruff of 220 grit will probably give you the look you want Also make sure to sand down any shiny surfaces so the paint will stick better. Alternatively you can use a "sand-free" spray that preps the surface to accept paint. It's just expensive to do some of the steps I use, but if you put on 3-4 light coats of black matte paint you should be ready to google drybrushing. You'll also want to check out rub-n-buff which is a wax and works great for disposable props, but not something you want to keep for a long time. The wax eventually wears off and so does the silver. Good luck and let me know how it goes!
How did you take apart the scope to paint it? I've been trying to take mine apart so that I could paint it along with my longstrike for a sniper cosplay, but I'm having issues with taking the end-caps off.