Having played plenty with shorties, ARs, and SMGs, I'd like to give sniper rifles a fair try. It seems to me that the variable zoom scope that you get towards the highest weapon xp levels is essential, but it is so tedious to get there! I'm hoping there is some strategy or gametype that I'm missing, that makes it less of a beating.
The variable zoom scope is not essential but it is a big plus on the larger maps. 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). 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.
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. 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.
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.
Personally I predominantly play sniper and the only maps I don't snipe on are Hard Hat, Dome, Carbon and Arkaden because they are too compact and busy. Maps like Output, Bakara, Mission and Fallen are probably the best sniping maps but if you're a beginner with the sniper be prepared to get taken out a lot by the experienced snipers until you've learned the sight lines and ways to move around the map out of the enemy sniper sights.
Bootleg, Lockdown, Resistance, Seatown, Underground and Village are all good maps for practicing sniping because they have some good sight lines but are less prone to being dominated by expert snipers like the aforementioned maps. Again for these mid size maps ACOG scope would work pretty well.
So I think the main difference between variable zoom vs ACOG is your preference for play style. Variable zoom scope will give you really long range kills but you need to be good at holding your breath and aiming, ACOG is super stable but shorter range. It works better if you like to move and snipe while variable zoom is better if you prefer to stay still and move infrequently.
Perk wise I use Sleight of Hand Pro as it is useful for fast switching to a machine pistol as a close quarters weapon. If you want to move around the map a lot (or camp in the open) blind eye may be useful to give you immunity from air support.
Secondly I use Hardline to get my killstreak faster, if you prefer like me to snipe from the same place for a while and move around infrequently it can be hard to rack up large kill streaks as enemies will often come flush you out of your favorite sniping nest. I usually go with Assualt Strike Package, either:
For the final perk either Marksman for the longer range enemy identification plus Marksman Pro will give you longer hold breath when you get it. Once I unlock it I usually switch to Dead Silence which is useful for sneaking around to flank enemies without them hearing your footsteps.
Equipment wise I usually have Portable Radar as my tactical (I unlocked it with a prestige token using Unlock Gear so I get it immediately rather than at level 75) which helps me spot enemies trying to sneak up on me. Paired with sleight of hand this usually gives me times to swap to my machine pistol at short notice.
For lethal either bouncing betty/claymore are useful for booby trapping an entrance to a sniper nest or just dropping in an inconspicuous place on a well travelled route to pick up extra kills.