Are you using you tactical crouch when you shot, to inches crit. chaste / damage done.
A head shot on an Assassin when you got manger-damage well kill then.
Don't forget that the rail gun does more damage the fever away you are from you target.
Lastly if your a bad shot or lagging making you miss (can also help tho e.g. targets frizz in place) try throw a shatter bomb at then if that don't move or it a never shipper It well land right no there head and not only well in do masses damage it also weakening them to any damage source now (for a time). Tho you will need to compensate for the Shatter Bomb's weapon drop i.e. it is effected by grivet, tho this is not to difficult to know how high/ low you will need to (over) throw to make it trave arouse the map and land right next to, on target to deal damage and is effete. Tho you may also need to taken in if you target is move left/right or away/towards you or if just move back and four i.e. you could throw three one to left, right and strata so that at lest one will hit, tho I do this and at most of the time ever two or all hit. Tho there is no need to do this on target that are not too hard to shot. Back to the lag thing tho, some times when I shot the shot shoots out of the rail gun at 65 degrees if not 85 or less.
All in all If even you or your target is leggy one well have an edge, tho who ever is sniping the lager will normally get the kill. But that is all if a lager kills you by other than sniping or Assassin attack that get doge still hit then it means if they did not lag you would of been kill faster.