The image explains how to mount the non-tilting barrel for the GSI Glock series.
Optional inner barrels have minute differences in outer diameter for each brand. To solve this problem and improve performance, a rubber O-ring is installed inside the outer barrel. When the barrel is mounted on the inner barrel, it is assembled a little tight. If you want a smooth assembly, it is good to use a little silicone-based grease on the outside of the inner barrel.
