The Mossberg 535 ATS is a pump-action shotgun designed and produced by the Mossberg Firearms company. Like most sporting shotguns--and in order to comply with local game hunting laws--the Mossberg 535 has a "magazine limiter" plug installed that prevents more than three shells from being loaded into the shotgun at one time. This plug is nothing more than a wooden dowel with rubber O-rings at both ends; it can be removed in order to load more shells without the need for any special tools or training.
Unload any shells that may be in the shotgun and rack the pump-arm back to ensure there are no shells loaded in the firing chamber. Clearing your weapon should always be the first step whenever you are performing any type of maintenance on a firearm.
Unscrew the cap at the end of the magazine tube. This is located immediately below the barrel.
Remove the barrel by pulling it forward and lifting it off of the shotgun frame.
Push the safety to the "on" position; shake the shotgun frame vigorously to loosen the magazine plug.
Load four shotgun shells into the receiver. The fourth shell will push the wooden plug out the front of the magazine. Grasp the wooden plug and pull it all the way out of the magazine tube.
Re-install the barrel and screw the magazine tube cap back on.
While removing the magazine limiting plug is not in and of itself illegal, many states have laws governing the amount of shells you can have loaded at any one time when hunting. Make sure you check these laws before you take your shotgun out into the field.