OK I got it now, I was not aware that it was an inner barrel replacement. I was thinking that you need to get a new barrel period. So I keep the outer barrel and swap the stock inner barrel for the TB one?
And someone talked about the spring what is the main difference between Mad Bull M100, M110 etc.... Seeing that they cost 15$ and the inner barrel 35$, mind as well getting everything, those are cheap upgrades, so it they make a difference I would be crazy not to buy them.