For what you require, DPMS would probably be your best bet since AFAIK, they are the one of the few that can offer you a threaded SS 24" barrel.  Be prepared to wait on a custom job.  Mine took six months but was well worth the wait.  Compass Lake Engineering also comes to mind if you have deep pockets but they may be faster than DPMS.  I have two of their barrels and they do superior work and you can go with a Douglas or Krieger barrel.