benjamin fortitude transfer port

hello guys well my fortitude just arrived after so many months waiting. I came upon a youtube video from southern gunner which caught my eye. The guy modded the fortitude and changed the transfer port but he doesn't know the size he install on it. I would like to do the same and gain a few fps more. so my question is what size of transfer port do i need to make the same mod. thanks 



https://youtu.be/vfYQ6gcB-EI







https://www.youtube.com/watch?v=vfYQ6gcB-EI