There is a difference.
The Police guns have the detent spring and plunger on the barrel support ring.
The NON-extended magazine Express does not have the detent, since it's designed to work with the new style magazine spring and magazine cap retention system.
The Express with the extended magazine does have the detent spring and plunger.
There are also other differences.
The Express barrels have a dull black blued finish, and have a rougher, less polished bore.
I "think" that Express barrels are made in two pieces, with the barrel brazed into a separate barrel extension and locking lug unit at the rear.
Police barrels are one piece, with a finely polished bore and a parkerized finish.
Remington does sell a standard Express 18" barrel with a bead sight. You'll have to contact them for ordering details.
