This shotgun here in CFPH is ONLY obtainable through the Daily Beginner reward for 7 days. There are no other means of obtaining it since its not available in the Item Shop, Mega Lotto (Black Market) Achievements, Events etc.
It could be obtained via reward box (daily/weekly rewards or online attendance). Since the new online attendance system took place, there's no way to get it (even for 3 days). Months ago, there were some "Spend Vcoin, Get Free Items" events that allow players who spend money in-game to choose any weapons (in list of course, Noble Gold not included). There's M16A4, but instead of being permanent, it only has 365 days.
This weapon was never announced or released publically in the game, but it was only available for 5 players to get the weapon through a special halloween event. 3 of them got the weapon, while I got the 2011 capsules that was never released. I was super duper lucky to win the weapon.
In CFNA, there was a list of the rarest weapons in the game released at the end of 2016 however, it never got updated. But one of the weapons which is still rare from that list is; AK-47-S-Gilded Skull. Only one person owns this weapon permanently.
Another weapon which could be considered rare is the M4A1-S Transformers Noble Gold. Only 10 players own this weapon in CFNA.