The 500 barrel system is different.
The barrels attach to the end of the mag tube with a large screw, instead of sliding over it.
A 590 barrel slides over the mag tube and is held in place with an end cap, like a Remington.
The advantage to the 590 system is that mag extensions can be added.
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”