[Helix_FR]

You need a federal tax stamp before you can purchase a suppressor. I believe its about 200.00 for the stamp alone.
From there you can purchase your suppressor which would be another 200.00 Im sure. Treading a barrel for it is not the job for the general machinist but a gunsmith with a lathe. A machinist cannot posses your gun w/o you around, you need a license to hold others firearms w/o them present. Threading a barrel will run another 100 or so.