An Excellent Choice!
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The Mossberg 500 is an excellent shotgun. I have one that served me well when I was an LEO and it is ready to go any time and any where. In some ways, I prefer it over the Remington 870 because of the location of the safety and the operation of the loading gate.
My suggestion while you are at the range is to pattern the shotgun; shoot whatever you decide to use as home defense ammo (00 Buck, #1 Buck, #4 Buck, etc.) at various distances that would simulate distances that it would be fired at home. For example, what is the pattern of 00 Buck or #4 Buck at 7 yards, at 15 yards out of your shotgun. You are looking for the tightest shot pattern possible at combat distances. Most folks use a 7" circle for patterning. How many pellets are there in a 7" circle at combat distance of 7-yards?
For my Mossberg 500 12 bore, #4 Buck could be devastating on someone's body at 7 yards and is my preferred round. I have a laser-equipped Mossberg 500 20 bore that I use for home defense and #2 Buck (when I can find it) patterns well at combat distances but #3 Buck is no slouch either and slugs are the preferred backup rounds.
If your Mossberg 500 came with a full stock and a pistol grip, keep the full stock mounted and, as one commenter noted, add a recoil pad (I use a Pachmyr Decelerator), which lessens the impact of the recoil and adds an inch to the Length-of-pull. With the full stock, you can add a shell holder for spares and you will have better control over the gun.