Madmacs, I've worked a regimental tam over using the method Jock uses, and it works just fine. I wear one regularly in the fall and winter, and it looks infinitely better after dunking, shaping, and drying than when it was brand new.

I assume you have a regimental tam, one of the newer ones that has a smaller diameter than the older one. (I have an older one, too, that I treated in similar fashion.)