On an airsoft gun if i add a longer outer barrel?

Question

then my previous stock barrel , do i need a new inner barrel ?

Answer 1

Yes, unless it's just a mock suppressor or flash hider. Then it just screws on. If you're actually replacing the upper assembly entirely, then yes.

Answer 2

It won't do anything but weigh more unless you just change  the barrel.adding a barrel shroud ( which is what a cover on a barrel is called ) is just cosmetic.