The STX barrel system from FX is known for its interchangeable liners.
These liners are centered and stabilized in the barrel housing by these 2-3 o-rings.

However, these rings can be lost/damaged when changing the liner.
So spares are always handy!

This set consists of 3 rings for the chosen caliber.

Drawings

As an example, these are the following rings on the technical drawings from FX:

• FX Impact Barrel: No. 9
• FX Dreamline (old type without superlight barrel): No. 4.6
• FX Maverick: No. 8.4

Exceptions

Not all FX barrels use these o-rings.
Exceptions are the old Smooth Twist barrels (e.g. the FX Boss) or the superlight barrels (later version of the Dreamline, Crown MK2).

Also, certain STX barrels from FX Standard are equipped with a Carbon Fiber sleeve instead of these rings. Examples of this are the FX Panthera, DRS Pro, etc.