Hmm.
Allow me to explain. You should have just bought an M79 if you wanted a dedicated 40mm, and you would have saved that poor M16 variant from the hacksaw. I assume it was a springer (the rifle), or something with a plastic reciever, which means it won't be that sturdy.
All in all, assuming you didn't spend alot on it (how much is your business only), it's a neat Mod. You could always carry it on your back to launch football grenades.
Either way, thanks for sharing it!