rails advice for classic army m15a4

[Gazzer1991]

Can anyone recommend/ point me in the right direction to getting a rail system for my m15a4. I’m mostly after the under barrel one in particular as I’m considering getting a grenade launcher when i can afford one.
I did find this one from zero one, however I'm not sure if it will fit or not.

http://www.zerooneairsoft.com/produc...oducts_id=3479