MGs don't have a barrel length requirement......
As said it currently only applies to the configuration of the weapon when its transferred. If the gun was made AFTER 1985 and has a barrel under 16" then it HAS to be on the MD Roster. Problem is determining when a MG is made, which involves a FOIA request.
Once you OWN the gun you can configure it how you want.
You can put a short bbl on it. The >16" limit is for a new transfer if you want to avoid the pistol roster mess. Since you already have it, no worries.