I bought an extra Remington factory barrel years ago for my 870 SM.  It's 18 or 20 inches and threaded.  It is a iron sighted slug barrel and came from the factory with their flush mount rifled choke.  I put a Remington "Ventilator" .655 Extended Turkey choke in it.  It is deadly with Federal's old Premium 3" 2 oz. 7 1/2 shot!!  Works good with 6's too!  Only way to find out is to try one you have and see what happens!  Good luck.