Thats exactly what I was just talking about doing about a week or so ago! The shorter inner barrel inside the standard outer barrel.. So it does sound quieter that way?? I have been debating it for about a month,lol..I just use the 430mm inner barrel in the 690mm outer. It works fine. I was going to use a normal lengthed outer barrel and a suppressor but the longer outer barrel works as a suppressor too so I'm not going to trouble PDI by having them pay for posting to and from Japan just so I can swap barrels.