FX AIR GUN SMOOTH TWIST BARREL

Offering it to others would likely be patent infringement. I am not sure FX owns the patent though because there have been other barrel patents in the past that employ a smooth and a rifled section. FX is unique in applying this to airguns though. FX refers to it as a proprietary process. Do a search on Google Patents. There is some interesting stuff there. 

Thurmond 
 
The term 'Smooth Twist' is part of the Patent that FX own, along with the system of making these barrels.
I would say you are quite free to attempt to make similar barrels* but you are not free to sell them as 'Smooth Twist' / 'SmoothTwist' or any other close variation of those words.


*FX bought the rights to Smooth Twist from Ben Taylor (Theoben) some years ago and spent many years (Blood, Sweat & Tears!) perfecting the system - do not assume it's an easy process. Getting it right involves sourcing VERY high quality tubing BTW, (Tolerances, ID, OD etc.)