The LEO/Govt ones have bayonet lugs and are threaded (available for pre ban rifles too).
The Civilan 'post ban' M4 barrels lack the lug and threading. The 14.5" version had either the AK or the Y-Comp muzzle brake permanently attached. The 16" M4 barrel can be had bare or with either muzzle brake.
As far as I know they only offer 1:9 twist rates, but could do 1:7 for large orders.