I started with the same gun (CA M15A4) and bought a CA M203 Long barrel mounted grenade launcher from Xtreme Tactics. I had troubles finding the mount kit for it. Make sure you get the right one. The M15 Carbine has a different mount kit. It will say in the box when you buy the launcher what the part number is for the one you need. I bought mine from Wolf Armouries. For the Upper Handguard I bought a G&P from Ehobby Asia on Ebay. It cost me $20 or something Like that. I had to file down the front of it a bit to get it too fit (it was too long) and It's still a TIGHT fit. But when all is said and done... It's still one sexay beast! Definatly though for the 70's get the A1 receiver
Here's mine. Don't have pictures of it with the new ACOG on it yet:
You need these 3 peices:
-CA M203 Grenade Launcher - Military Type (Long)
-CA M15A4 Rifle Mount Kit
-Upper Handguard