FEINWERKBAU 300 FRONT SIGHT INSERTS
The original FWB front sight inserts or apertures is 18mm in diameter, which is similar to what other manufacturers use. Sometimes called M18inserts, these alternative inserts fit into the threaded front sight’s sleeve of the 300S. The inserts are clamped between the clamping sleeve and the shoulder inside the sight to prevent movement. FWB apertures are unique
from other target air gun makers in that the side tabs are offset a little different in how they fit into the slots of the front sight housing.
Champion’s Choice, and many other target supply houses, offer a set of ten 18M type acrylic inserts that are just round without the tabs and are usually advertised to fit FWB, Diana, Walther, and Anschutz air rifles.
While talking about sizes of these inserts: many older guns use an “M17” (17mm) sight insert that is no longer available and the latest air rifles use a M22 (22mm) inserts.