I forgot to answer your first question. There is no way to add a magazine extension to the Mossberg 500 or Mav 88. The Mossberg 590 will take an extension due to the different design of the magazine/barrel locking system. If you turn the screw/bolt at the end of magazine you can take the barrel off (pull back forend halfway first). You can see that the part the bolt screws into closes off the magazine tube. Other shotguns use a larger cap type screw on the end of an open tube, which is why they can be extended.
The only way to add rounds is a side saddle or butt cuff. But this adds weight and bulk and some of the things that makes the 5+1 Maverick 88 great is the short length, light weight, and slim profile. Yes, I have other shotguns that hold 8 rounds but they don't have the maneuverability of the 18" Mossberg.
And since you violated the unwritten rule of posting pictures of your new gun, I'll help you out
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I would keep the magazine tube full and not worry about long term spring compression right now. I think it's better to have it full and consider replacing the spring in 5 or 10 years, if needed.
I purchase 5+1. I just check and yes I can hold 5 in magazine and 1 in chamber.
I'd point you to the shotgun forum that I posted on for a few years but that isn't exactly kosher to promote another forum on this one. There were a lot of shotgun experts there. I don't think I ever saw one say store a pump gun with a round in the chamber.
I forgot to answer your first question. There is no way to add a magazine extension to the Mossberg 500 or Mav 88. The Mossberg 590 will take an extension due to the different design of the magazine/barrel locking system. If you turn the screw/bolt at the end of magazine you can take the barrel off (pull back forend halfway first). You can see that the part the bolt screws into closes off the magazine tube. Other shotguns use a larger cap type screw on the end of an open tube, which is why they can be extended.
The only way to add rounds is a side saddle or butt cuff. But this adds weight and bulk and some of the things that makes the 5+1 Maverick 88 great is the short length, light weight, and slim profile. Yes, I have other shotguns that hold 8 rounds but they don't have the maneuverability of the 18" Mossberg.
And since you violated the unwritten rule of posting pictures of your new gun, I'll help you out
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Not to be an *** by my Mossberg 500 Cruiser Breacher can take magazine extensions. Certain models do have the screw on end caps like the 590s do. Choate Machine makes them, among others.
The shotgun isn't my primary defensive weapon but I typically keep shotguns with the chamber empty, magazine full, hammer down, safety off. The Mossberg has the forend release in an easy to reach location so hammer down isn't as important as on an 870, in my opinion. I know you're new to the Mossbergs/Mavericks but look into installing a screw into the trigger group to give you a nicer trigger. The S&W sight screw works. I think it's either 6-48 or 8-40 threads.
Also as to the 5 round capacity, it will take 5 3inch cartridges but I use 2 3/4 and it makes it easier to get the last one in there and there is also more room for my thumb when pushing them in with the shorter rounds. Also compresses the magazine spring a little less with 5 2-3/4 shells which may/may not add to the spring life.
PS.
If you PM me your address I'll send you an extra fiber optic bead sight I have