For rifles that have a shrouded and/or recessed barrel they are often not perfectly centered, and giving that little extra space doesn't substantially increase noise, but reduces the probability of problems. Sometimes the pellet doesn't even clip, but it almost does and the air pressure influences trajectory (and accuracy) in a bad way.
We've changed our small bore product lines so that moderators are made in .177/22, .22/25 and .25/30. If you were direct threaded to a .22 cal barrel, I'd be comfortable recommending either the .177/22 or the .22/25, but if you had a shrouded barrel I'd definitely recommend the larger .22/25.
I hope this helps.