Hello Cherry
You will also see barrels with AC being used- again autococker shortened. Depends on what you want from you barrel, Smartparts do a barrel system called Smart parts freak. This system has a barrel back, an insert and a barrel tip. The inserts are interchangeable depending on paint size. You can get these 2nd hand off the P8ntballer for around £50.
There are other makes like stiffi (carbon barrels), Dye ultralite, CP (custom products), deadlywind (carbon barrel that uses the smart part freak inserts) and many more. You can pick up the backs for your ego theses are called shaft barrel backs there are 4 shaft barrel systems. The newer shaft 4 has a different thread system to the others.
Regards
Ants