Which do you prefer, magnum or express buckshot? Here are their differences: (I got most of this info from the wiki and battle buddy)
Magnum Buckshot: 400 damage (9×50), -15% accuracy, +115% recoil, 385 m/s muzzle velocity.
Express Buckshot: 315 damage (9×35), +15% accuracy, no recoil change, 430 m/s muzzle velocity (Additionally magnum has a 20% chance to cause a light bleed and a 10% chance for a heavy bleed but I won't be factoring those in as I dont think they cause too much of a difference, feel free to correct me if im mistaken).
What these numbers mean: (assuming you hit all pellets on one body area) both will kill a pmc in two shots to the legs, either both shots to one leg or one shot to each leg. However, magnum buck will kill in one shot if all pellets hit an unarmored stomach. Seeing as they both do at least 35 damage a pellet, they will both kill with one pellet to an unarmored head area. As far as I know, most people run magnum buck, but I think there's a case to be made for express. Assuming all pellets hit, other than the stomach they both have the same shots to kill, but lets say you miss a pellet or two, which is bound to happen. For starters, express has a +15% accuracy bonus compared to magnum bucks' -15% and an additional 45 m/s velocity, so you're more likely to land all your pellets compared to magnum. Additionally, magnum has +115% (!) recoil, so if you are using a semi auto shotgun it'll be much harder to land a follow up shot (probably not as much as a problem with a pump action). I've been using magnum for a while, I think i'll give express a shot.