Does anyone know if model 500 barrels can be used on 590s?Also does the tube extention for the 500 work on the 590?

No the barrels and mags are diffrent on 500 & 590's

Well, there's no such thing as a magazine tube extension for a Mossberg 500, so let me answer this a different way.
The 5-shot Moss 500, the 8-shot Moss 500, the short 590 and the long 590 ALL have interchangeable barrels and magazine tubes, but you MUST change [b]both the barrel and magazine tube[/b] for it to work.
Example: My 8-shot Mossberg 500 began life as a 5-shot 500. A buddy of mine wanted a 5-shot also, so I paid the difference in price and he bought the 8-shot one. I then swapped the barrels, magazine tubes, and magazine springs. I also swapped mag followers, but that was unnecessary. Then I had the 8-shot and he had the 5-shot, but we both kept our respective receivers and serial numbers.
In the same way, the 590 parts can go on a 500, and vice versa. You just have to change the whole front end, since the 500 and 590 barrels attach differently.