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 highest tier bolt sniper rifles (EM 4 Longshot) because of higher bullet velocity and the fact that the range they do their max damage at is greater - meaning that they cane oneshot an enemy with a headshot at ranges the Bolt Driver can't. The LA80 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 LA80 has scope sway, though, meaning you have to hold your breath to steady your scope, thereby limiting the time you have to aim your shots precisely. 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
Aim for the head. Headshots with bolt sniper rifles are oneshot kills up to a certain range depending on the sniper rifle
When enemies are nearby and you are alone, don't spam your cloak, enemies are able to locate you
Use your sensor darts if you want to know where enemies are, don't rely entirely on them since they can be tricked
Cloak doesn't offer total invisibility so you can indeed be seen by enemies at close and medium range. Visibility goes Standing>Crouching and Moving>Walking
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. This way you can get more kills (personal experience) and the need for a suppressor is considerably lower = less time to learn bullet drop, more kills, less frustration.
Have a look at this http://planethead.info/weapons/heavy-weapons for in-depth information about guns and gunplay mechanics