First of all, I prefer playing objectives. I don't like sitting far away just to get one of those cool marskman headshots...so I might not be the ideal person to answer this.
Anyway, I prefer not using bolt action in this game but when I do, I like it with good scope. It's meant for headshots afterall.
I like my semi w/o huge scopes. In fact, Mod 0 and EMR (I prefer EMR) are pretty good with ironsight. You can also slap 1x scopes and they'll be just as good too. This setup is ideal for taking out still enemy in mid range. They won't stand a chance. Since you don't have a sniper scope glare (b/c you're not using huge scopes) they won't be very afraid of you...well until you put three quick shots on 'em. For anything staying still at range (not very far away btw), I always spot and then shoot 3-4 quick shots at the spotted point. They'll drop easy. At close range, pistol is your friend. And MAV (put it stationary closeby) or TUGS will help a lot. Watch the minimap, get around, and get a drop on them first. If you don't get a drop first, then getting a kill with semi becomes harder unless you've a good cover.
hope that was somewhat helpful. although i sense you already know what i said above.