All the 22 FTTs are always listed as 5,5mm. That's European for 5.5 mm which is generic sizing for 22 cal. FTTs come in more finite sizes. 5.53, 5.54 and 5.55mm. Those sizes are listed on the back label. Some Weihrauch's respond better to one size more than another. I have the best luck 5.54...