This is a barrel for both single and dual MK8 extruder with a PTFE tube inside (Note:3mm model has no PTFE tube inside). 

Please note that the 1.75mm MK8 extruder is not compatible with the 3mm one, and you can not convert your 1.75mm extruder into a 3mm one by just changing the barrel.  please confirm your expected diameter before placing the order.