The SAS-R is a nice weapon for both cqc and longer range engagements although especially for cqc you need some decent reflexes, so it might be not the best for a newbie (except if you already have considerable sniping experience in other games). Your starter sniper weapon the Bolt Driver is another decent option for medium to long range engagements, although it gets outclassed at the latter by the high tier boltsniper rifles because of higher bullet velocity and the fact that the range they do their max damage at is greater. The Bolt Driver is a compromise between those two categories: not perfect for close range engagements and not for long range ones (but imo the best engagement range for a sniper wanting to get kills is exactly where this weapon shines). Other than the cqc bolts (SAS-R/Ghost/TSAR-42), the bolt driver has scope sway, though. I can not talk about the Railjack from personal experience since I have never used it before. But looking at the description saying it needs you to "warm up" before a shot this seems to somwhat offset the fact that you don't have to lead your moving targets and/or compensate for bullet drop due to high velocity. When shooting at people standing still for a longer period of time - yes, those exist and quite a lot of them are other infiltrators - this wouldn't matter ofc.

Some general tips: Use your cloak to bring yourself into position for a shot Never stand still, except when you are shooting at an enemy Try to hide behind an obstacle when you are not shooting at an enemy Do not cloak and stand still where you are. I kill quite a lot of infiltrators who think their enemies suffer from a loss of short-term memory Change position after every shot at an enemy even if it's only a few metres Don't go on lone wolf sniping. Try to stay somewhat behind the main force of your faction. Hide in the mass.