If it truly has a 5.3oz buffer, then you don't need to buy another one unless it doesn't function correctly.
As @shooterrexsaid above, there is no relationship between a rifle gas system and a rifle buffer. The same can be said of carbine systems. You can use a carbine buffer system with a rifle length gas system and vice versa. They do not need to be the same designation, only balanced gas and recoil impulses.