"W" stands for "Wrgebohrung", which means "choke". In the case of a side by side double shotgun, the right barrel, which is controlled by the front trigger as a rule, is more open, the left barrel tighter. In the case of a 16 gauge barrel, the minimum bore measurement at the muzzle 16.80mm, the maximum 17.20 mm.
Determining the degree of choke cannot reliably be determined by measuring the bore. The only reliable method involves shooting the gun at a patterning board and counting the shot inside a 30" circle from various ranges.
A full choke should put the entire shot pattern inside the 30" circle at 30 yards, A 1/2, or modified choke should place 83% of the shot in the circle and an improved cylinder should achieve 72%. A one ounce charge of number 8 shot contains 450 pellets.
