Standard buckshot, magnum buckshot, 3/0 magnum buckshot and slug 20 gauge shotshell can be crafted at a Reloading Bench. A single shell requires the following ingredients:
Magnum shells are standard buckshot shells fitted with a higher internal powder load, increasing velocity and stopping power. The downside is that they cause more damage to the firearm used due to the higher overall pressure, requiring more regular maintenance.
20 Gauge, Slug
20 gauge slug compared to .22LR bullet
Slugs contain a large lead projectile promoting better accuracy and kinetic energy over a distance. Slugs enable a shotgun to engage targets at a further distance.
20 Gauge Round (Junk)
A round made from refuge materials, their shoddy quality increases wear on the weapon by 1.5x and deals 25% less damage to targets.
20 Gauge, 3/0 Buck
A sub-type added with the add-on Gun Runners' Arsenal, these loads sacrifice quantity for quality, in other words, there are fewer projectiles overall but they're heavier and pack a bigger punch.
20 Gauge, 3/0 Buck Mag. (Hand Loader)
A sub-type added with the add-on Gun Runners' Arsenal, these magnum shells use the larger 3/0 shot size, but with a greater powder load resulting in higher velocity. The Hand Loader perk is required to craft it.
20 Gauge, Pulse Slug
A sub-type added with the add-on Gun Runners' Arsenal, these slugs cause more damage to power armor and robots, and the target gets an extra kick from an electromagnetic pulse triggered by the impact.
3/0 Buck Rounds only have a x1 damage multiplier yet standard 20 Gauge Rounds have a x1.2 multiplier and Mag. rounds have x1.3, including the 3/0 Mag. rounds (these multipliers were added as part of patch 1.5). As a result, 3/0 Buck actually inflicts less damage than standard rounds while inflicting more damage per projectile as the damage is divided evenly among them.