I have had a freak kit, a freaked phantom barrel, a freaked palmers barrel (not really recomended
...), and a factory freaked barrel on my Retro 7.
I use the wonderful Brass inserts
that Tymcneer makes.
I personally like using my stock barrels and boring them for freaks when I can, keeps the lines of the gun that were designed for it.
As long as the gun has enough meat, then having a bored barrel wont be an issue. once the barrel is screwed in, the insert is held in place.