2 rounds of 12 gauge slugs, used in the Double-barreled Shotgun. They contain a single projectile which can accurately hit a target 250m away, and are powerful enough to kill a zombie with a single shot. You cannot merge depleted piles, and you cannot use the 2 round magazine in the M1014 or Remington 870 (Flashlight). However, 4 stacks can be merged into a single 8Rnd. Slugs. Conversely, 8Rnd. Slugs can be converted into 4 2Rnd. Slugs by right clicking them and choosing the split option.
If given the choice between slugs or 2Rnd. Pellets, slugs are always the better choice. Pellets are ineffective past 40m, putting the player at a significant disadvantage in many engagements. However, at closer ranges, like the insides of buildings, pellets can be used to get multiple kills or headshots with a single shot.