This is a great shotgun forend light and I highly recommend it for your Mossberg. However you need to be aware of the limitations before you buy:
-If you have the 590A1 with the bayonet lug, you will need to grind off about half the lug before mounting the forend
-You will need a 12 Gauge Shotgun Forend Wrench to install the forend. If you do not have one (~$30), a gunsmith should be able to do it for you for around $20. So call and get the price from your smith as it may be cheaper to purchase the tool and do it yourself (midway "dot" com part# 723227 $22 +S&H)
-If you change your shotgun stock, it will change the dynamics of recoil and the back end of the light may impact your hand holding the forend. This can be compensated by changing your grip a little
-This model is not the one that you can disable the light with a switch. However, it does include a panel that will block half of the momentary button bar
NOTE: I only gave it four stars as it would be fairly simple for SureFire to modify the forend to allow fo the lug.
Enjoy!