First off, a small rundown on buckshot pellets
per shotgun vs slug rounds
per shotgun as seen on wikia (Buckshot / Slugs):
Buckshot Pellets / Shotgun / Damage based on Distance
----------------------------------------------------------
11 pellets SPAS-12 18.0 - 6.0 / pellet
9 pellets SAIGA12K 18.0 - 6.0 / pellet
9 pellets DAO-12 18.0 - 6.0 / pellet
10 pellets M1014 18.0 - 6.0 / pellet
12 pellets 870MCS 18.0 - 6.0 / pellet
7 pellets USAS-12 18.0 - 6.0 / pellet
8 pellets MK3A1 18.0 - 6.0 / pellet
12 pellets M26 MASS 18.0 - 6.0 / pellet
// Headshot Bonus : 2.40x
Slug Rounds / Shotgun / Damage based on Distance
----------------------------------------------------------
1 round SPAS-12 100 - 40 (pump-action)
1 round SAIGA12K 75 - 37.5 (semi-automatic/auto)
1 round DAO-12 75 - 37.5 (semi-automatic/auto)
1 round M1014 75 - 37.5 (semi-automatic/auto)
1 round 870MCS 100 - 40 (pump-action)
1 round USAS-12 75 - 37.5 (semi-automatic/auto)
1 round MK3A1 75 - 37.5 (semi-automatic/auto)
1 round M26 MASS 100 - 40 (pump-action)
// Headshot Bonus : Insta-kill?
That being said, in my opinion, in close quarters when your reticle is mainly on the target when you're using buckshot (especially on the area above the waist), its usually a instant-kill for me, and I'm not even aiming the weapon.
But if I chose to aim, or have my reticle generally focused on the enemy torso at close range, I would be able to also instantly kill someone at close range.
From your question, however, it is also debatable on your definition of "close range", which is usually 25metres and below in-game (for me).
I would also -- if given the limited choices of a buckshot and slug shotgun only in the entire game -- choose the:
Slug
for a long range engagement such as a long-reaching room or a wide area, because you can at least aim and be guaranteed to kill something in here.
Buckshot
for tight corners and small rooms within 25m because its a confirmed kill usually at such low ranges.
Hope that answers your question.