I know this has been discussed before, but I was unable to find any threads via search.  I did find some threads from other forums, but I wanted to see what the responses were here.  On other forums, I read that the only difference was the plastic trigger assembly/guard on the 500 and 590 vs. the metal trigger guard/assembly on the 590A1.  Also, I have read that the barrel and mag tubes are different on the 500 and 590/590A1.  Is there any other differences, especially internal parts like extractors, ejectors, etc.?

The 590 will hold one more shell than 500 with barrel length being equal. I have a 590 it holds 9 rounds,  my 500 only holds 8 rounds.

The 590 is an upgraded, stronger version of the Model 500.
When Mossberg tried to sell the Model 500 to law enforcement, they had problems with the gun failing to stand up to the abuse police guns typically get.
So, they upgraded the 500 to a tougher gun that would stand up.

The 590 has a Remington 870 type magazine and barrel mount system that allows easier cleaning of the magazine tube.  This also allows using or not using a magazine extension without having to change the barrel and magazine tube.
The 590 barrel is thicker.

The Model 590-A1 is a 590 upgraded with some stronger parts that would be acceptable to the military.

The basic receiver and trigger group assembly is the same on the Mossberg Model 88, 500, 590, and 590-A1.

It's easier to swap replacement barrels for the 500 series.  The 590 requires additional parts to swap different barrels.

How so? which additional parts?

I know that you know your shit, but I believe you are mistaken about the heavier barrel. The 590 does not have the heavy wall barrel, but the 590A1 does. I'm 99% sure this is the case with all the 590s/590A1s because I noticed that while shopping for a 590A1. Every 590 I looked at did not have the heavy wall barrel.



 

Actually, the first Mossbergs taken into the military were the M590, and NOT the M590A1.  Might have upgraded and renamed them since my day but that's what was around in the late 80's.  Interesting note is that the first 590s we seen had the regular bead sight but later models used a ghost ring sight but not like the ones you see Mossberg selling these days.  I guess the closest sight I can think of would be the MMC adjustable ghost ring sights but I'm not sure if those are being made any more either.

Correct, the 590A1 has the heavier barrel, metal safety button, and metal trigger housing.  The 590 has the standard barrel thickness barrel, and a heat shield that doesnt fit the A1.

Oops, got the barrel thickness wrong.