Ugh. Why do people post when they have no clue what they're talking about?
To the OP, the YHM can will sound like a LOUD .22LR. Depending on your own ears and your shooting environment it may or may not still make your ears ring. If you're shooting outdoors in an open field the chances are better that it will not. If you're shooting in an enclosed hallway it will almost certainly still hurt your ears.
I would always advise anyone interested in a 5.56 suppressor to see one and shoot one first. Almost every buyer of 5.56 cans that doesn't is disappointed with their purchase.
To the rest, there is no "class 2" license in the sense of a license needed to own a suppressor, there is no "list", and there is no tax stamp for a flash suppressor.
To own a SOUND suppressor (or "silencer"), assuming they are legal in your state...
1) Find the one you want (best if you find a dealer with it in stock)
2) Pay the dealer
3) Fill out the forms (to include CLEO signature, fingerprints, etc.)
4) Send all documents to BATFE with $200 check for the tax
5) Wait
6) Wait some more
7) Get form back from BATFE approved and with a nice stamp in the upper right hand corner
8) Pick up suppressor from dealer
There are various nuances to the process that can be encountered if ordering the can from out of state, using a corporation for ownership, using a trust for ownership, etc. but that's the basic process. Also, many dealers will fill out the paperwork for you which can be a nice service but you still need to check over the forms before you send them in, ask me how I know.:banghead: