How to connect a silencer to a potato gun?

i have a 2 inch barrel and would like a silencer i can just put on the end of it but i dint know what size coupler i would need what im doing is getting a 2 inch piece of pvc and putting it inside a 4inch pvc but i wont the 2 inch to connect to my barrel so what size coupler do i need to get it to connect to the barrel and how do i get it to stay in place inside the 4 inch pvc like what size reducers or couplers do i need