If you got the black one, not the CT clear one, I would recommend you drill 2 holes on the receiver and use one of those M16 heat-shield RIS bolt-on things.
or if you have a M16 handguard, or an RIS with holes between the notches, just bolt it straight on.
It's not going to be accurate either way, since the receiver is curved.
If you have the CT one, you had better bondo the shit out of it, cause it'll break into little pieces when you cock the thing.
Or maybe you can hack apart an M203 if you are rich?
http://redwolfairsoft.com/redwolf/ai...l?prodID=22569