Why would you buy an after market barrel for $118 when you can buy a Remington Factory new 18.5 bead sight for $101 from Remington and even less on Gun Broker? The 18.5" parkerized 870 Police barrel is $99 on gun broker and it is made at the Remington factory in Ilion New York. There is also the 18.5" matte finish bead sight barrel on there for $118 and it is a Remington Factory barrel.
That is a better deal then putting a Mosberg made barrel on your Remington.
As for the detent ball on the barrel. It has nothing to do with your mag cap as you have an internal retention system and can use the same mag cap. It will simply push the detent ball down and out of the way. You do not need anything but the barrel.
J.D. McGuire, Owner
AI&P Tactical, LLC
Remington Factory Certified
Law Enforcement Armorer