[Theohazard]

A washer like that is likely to mis-align the can. And it might misalign it enough to cause baffle strikes. I highly recommend never using anything between the silencer and the shoulder that isn’t specifically designed for use with a silencer. But if you use one anyway, make extra sure to check alignment every single time you screw the can on, not just the first time. And I wouldn’t use a wooden dowel, those aren’t precise enough.