Are you talking about the VSR? If so, do you mean extend the barrel through the silencer, or have a piece of barrel in the silencer, that is then detachable?
If you mean the first, then there'll be negligible range or accuracy benefits from having the longer barrel, you'd probably gain more from having the silencer filled with foam to quieten the gun.
If you mean the second, then, no offence but, I must say that's an awful idea. The barrel won't be consistent, it won't align perfectly every time, and will result in worse range and accuracy.