The other benefit of using it all the time is that you'll learn all the crazy sight lines and get better at quick scoping which are both important to getting more kills and thus more XP. Using it with an ACOG will make it more versatile and usable on smaller maps but on smaller maps you're more likely to take a shotgun to the face so probably not the best of ideas unless you are a lethal no-scoper. In the long run the best way to level up a sniper rifle is simply to practice with it until you get good enough that you can use it on most maps so that you are always racking up XP with it. If you don't want to have to camp and snipe a lot another strategy is to equip the ACOG scope on one of the above rifles and use it more as a long range assault rifle, with the Kick proficiency and the ACOG they can be remarkably stable and accurate.

Other useful attachments are extended mags if using a semi-automatic like the Barret 50 cal or the AS50 (bullet spam can make up for bad aim on occasion). The variable zoom scope is not essential but it is a big plus on the larger maps.

Call of duty mw3 sniper rifles