If we leave out specials in primary slots (universal remote, no land beyond) then theoretically it would be hawkmoon. It is capable of a 1shot if all lucky bullets proc.
Last Word. Most OP gun right now.
You can checkout this sheet(saw it on reddit) - https://docs.google.com/spreadsheets/d/1L7FV9l4hXo2tHD3XlFUp854XvJIwTKFIPvASLduPPS4/htmlview?sle=true#gid=0
For non bolt action ranged primaries it's the last word and thorn. Followed by hawkmoon, red death.
Technically it's the no land beyond and universal remote but they aren't reliable.
Thorn, last word are highest
No land beyond and universal do not count,
Hawkmoon does not count either, because it is not consistent. It's abillity to one shot is not consistent
Not counting Universal Remote or No Land Beyond, the answer is Hawkmoon. [b]IF [/b]you stack two bonus shots it is a one shot kill. Outside of that two hip fire head shots with The Last Word will kill in 0.26 seconds.
No Land Beyond. Because it can one shot people consistently.
Hawkmoon can potentially one shot people if all three lucky shots roll on the same bullet.
Universal Remote (I think) can one shot as well.
Everything else for primaries is two shots minimum. Thorn, Hawkmoon (average case), Red Death is two bursts, Ill Will, etc.