I don't think it's a new type of gun but sorta like a relic. If you look closely you can see that on the titans helmet their is a red symbol. If you look closely at the hunter their is that same symbol. Knowing this I say they are wearing some kind of set of armor. The gun also has the same color scheme as the armor. My guess is that this is something from the prison of elders. The gun is something you get from the awoken to go into the prison of elders. Like you are a guard and you are issued a specific uniform and a specific weapon.