Local gunshop has a post ban AR15 with a threaded barrel and a muzzle break installed. I commented that I thought it had to be permanently installed (welded, etc...). And Jim remarked "it was legal because it had non-standard threads". Not wanting to open my mouth and prove how ignorant I was, I left it at that. Can someone please explain standard and non-standard threads and how this can be legal? If they're making muzzle breaks in non-standard threads, what about the evil flash surpressor? What is standard threads and what is non-standard? Is the non-standard the new standard? Hope these questions aren't too dumb. Thanks, Matt.