BudsGunShop.com - capacity

Frederick, straight out of the users manual "With the magazine three-shot adapter (plug) removed, your shotgun holds four 2¾" or 3" shells in the magazine." winchesterguns.com/content/dam/winchester-repeating-arms/support/owners-manuals/10-406-wra-sxp-om.pdf

Frederick, 4 rounds in the tube with 3 or 4 inch shells. There is only 1 inch difference in (4) 2 3/4 and 3 inch shells so a 5th 2 3/4 is not possible.