midwayusa.com/product/1005529614/mossberg-magazine-tube-8-round-mossberg-500-a-12-gauge ......
brownells.com/shotgun-parts/magazine-tube-parts/magazine-tubes/mossberg-500-magazine-tube-8-shot-12-ga-prod19943.aspx .....
.............. and other places as well. Note that a 20" barrel is recommended for the 8 shot extension, as it is just over 19 inches in length, and will protrude out past the end of the 18.5" barrel. Also note these are new magazines - not extensions. This is the recommended way to 'extend' the capacity of the 500.

Shaun, I looked up the tube extensions but they are for the 590. On the 500 You can not extend the 5 shot tube , you have to replace the tube with the longer version. You also have to replace the barrel to match the longer tube. You cannot extend any of the 5 shot 500s without a barrel change. The cap is welded pressed to the barrel, preventing any extension of any kind. ----------- gggaz.com/mossberg-590-magazine-tube-extension.html