From my experience, mock suppressors will never actually, 'surpress', your rifle; they are purely for the, 'tacticool', look on your rifle.
I have tried a longer inner barrel in a smaller outer barrel, all you have to make sure is that the supressor will fully cover the inner barrel which is sticking out the end of the outer. The only downside to this is that you will always have to have your supressor attached to the rifle otherwise you will have your inner barrel sticking out the front of the rifle making it susceptible to damage and your rifle will look incredibly stupid.
The only performance change you will really get is the one's created by the barrel itself. i.e. you will get your increased accuracy and fps via the length, bore and diameter of the barrel, but the supressor will only serve to make the rifle look visually better, and may make a slight difference on the sound of the rifle, but not by much.
Have you ever heard a TM G-spec without it's supressor attached? It sounds almost exactly the same, meaning there isn't really any need for a mock supressor in airsoft as you are not suppressing the expanding gas which pushes a bullet out of a barrel to hit it's target.
However, saying all of that, I have heard of people who have brought mock suppressors and have modified them to have suppressing qualities of RS suppressors. Although depending on where you live, the law on modifying/owning a supressor which possesses suppressing qualities changes. As in California, I am aware that owning a supressor which isn't licensed can land you 11 years in jail, and if you have modified your supressor to posses these qualities then you have no defence so it's probably a good idea to let the supressor, if you wish to purchase one, stand as a 'tacticool' add on to your rifle.
Hope this has cleared some stuff up, any more questions just let me know!
Zero