18.3 should be the bore diameter in mm, not barrel length.
On European shotguns, you'll often see two numbers on each barrel, one indicating the bore diameter, the other indicating the choke.
For example, on a Sauer Aya shotgun I just checked, the barrels are marked:
Choke
17.3
18.6
Choke
17.3
18.6
This indicates that both barrels have full chokes.
When in doubt... a tape measure is a useful tool.
(^^I just wanted to use that emotion)
When in doubt... a tape measure is a useful tool.
(^^I just wanted to use that emotion)




























