You can't be playing Call of Duty MW2 onwards then because a Sniper is insta death 93% of the time and an AR is about 2-4 shots depending. Nothing but explosions, snipers, throwing knfes and normal melee can kill instantly, though the last two are ridicilous..
Barrett M107 (that one?) isn't, but then it's based on an AMR, Barrett M82. Remington MSR is based off M24 (and that in turn is based off Remington 700), if I remember correctly. AS 50 is an AMR. No idea what "L118" is even supposed to be.

As I wrote earlier - I primarily play ArmA 2 nowadays. The CoD series was always rather too silly for me.