I've been here before with some posts but yesterday gave me a better example. This year I purchased a Haenel 28 that is in new condition so the barrel is as you would have found it in the 1930's. Since this is my first break barrel Haenel it gave me the opportunity to try pellets by pushing them through the barrel. I first tried Crosman Premiers, which are very good pellets and in this country sometimes the only pellets in locals stores. These were 22s and they started by going down into the barrel when loading. Pushing them through offered no real resistance, and thus very little if any engagement of the rifling. So I assume the barrel and the pellet are exactly 5.5 mm. I did try other pellets that had skirts that offered significant resistance, obviously ones better suited for this gun.

Since Crossman Premiers are used so much over here I assume modern barrels are actually bored so the rifling is slightly less than 5.5 mm?