I threw my 12" bbl into the freezer, dropped the end of my receiver into boiling water, and jammed them both together real quick. Worked like a charm, and as a plus its nice snug fit.
I dont plan on switching back to an 18" barrel though.
Edit: and If you actually read that nice piece of paper that your registration Certificate comes with, you only need to inform the CFC if:
Any mod that affects its classification: non-rest, rest, prohib (the 12" barrel does not affect class, it remains non-rest. therefore does not need new cert, therefore no notification to the CFC)
Any mod that results in the firearm ceasing to be a firearm.
Any mod to a firearm or receiver that allows it to discharge ammo.
Any mod to the type, action, calibre or guage of the firearm,
Any modification to an altered automatic firearm