PVC width for Retaliator/CS6?

I want to turn my Retaliator into the M4A1 from MW2. I haven't painted it yet, but I want to know the width I should use for a PVC pipe barrel extension, or if I should be using PVC at all. I measured the width of the barrel and it's somewhere between 1/2 and 3/4 of an inch. Should I modify the barrel and stick a 3/4 inch into it or go with the slightly smaller 1/2 inch? Thanks