Barrel for HK FP-6 Shotgun

I am looking for a 20 in barrel and the rifled barrel (i have heard that this barrel wasn't put into production but i have seen one before that was for sale but this is before i had the gun) please let me know if anyone knows how i would go about getting one or if anyone has one that they would sell please let me know i have the 28 in barrel and just looking for the others.

Thanks