Mossberg 500 Marine coat slug gun

Was told on buy this gun that its magazine capacity was 6. Come to find out that I can only hold two in the tube and one in the chamber. Thats fine for hunting and would never think of loading 6 shells to go hunting with. But I wanted the gun to double as a home defense gun after I obtained a smooth bore barrel for it. Can anyone give me any advice, since removale of the barrel at the end of the tube reveals a threaded end that can't be removed with my knowledge, once again thanks for your time

you have to take the plug out. on alot of mossbergs, you just have to unscrew the magazine cap and shake the gun a little with the barrel pointed at the floor. a dowel rod should drop out of the magazine. put the cap back on and you can now hold your full capacity.