OK, trying to help out here some.
Barrels under 16 inches long being installed on a rifle, must have a permanent extension pinned and welded (usually a long CAR flashider or Phantom comp/hider) to bring it to 16 inches overall length of the barrel to comply with federal law unless you are exempt with a cancelled tax stamp much like the one that caused our ancestors to dress up like indians and throw the Kings tea into Boston Harbor, or are using it in an official manner. ie., law enforcement officer.
preban configuration barrel 16 inches or longer--
are all threaded
postban configuration barrel 16 inches or longer--
not threaded. I've also seen these called
competition and varmint barrels
Under 16 inch barrels almost always have threaded barrels. Only way to legally use on a rifle is to apply and be approved for a license from the NFA branch of the ATF. These short barrels are usually called pistol barrels or SBR barrels or machine gun barrels or a host of other terms relating to law enforcement and military use only.
Bottom line, if you're not wanting to jump through hoops to obtain an SBR license (cancelled tax stamp), as long as the minimum overall length of the barrel is 16.0 inches, you could get a preban barrel and change whatever flashider or comp you want as often as you like.
Barrel threading for 5.56/.223 AR15's is 1/2-28 threads per inch.
*Not all comps and flashiders are this thread pitch as there are a lot of alternate calibers for AR15's.
In some states and munincipalities, threaded barrels are not allowed for civilian ownership. Most states don't have problems with this.
You can check to see if threaded (preban) barrels (and other things related to assualt weapons) are legal in your state by clicking here. Hope it helps
OA2.org :: View topic - Federal/State Gun Laws