Why does it work? Would an 8mm silencer work better? Would a .45 cal silencer work as well?
Let's get back to the "availability" part. 8mm might work too, but can you find one? My guess is that .308 (or close) and .355 (or close) isn't a big difference when considering the properties of a silencer. We're only talking about 5 hundredths of an inch (or so). 9mm works.