Since the Magnum Marine uses the retention spring and plunger in the front of the barrel support ring, you'd be best in getting a new barrel fitted with the plunger parts.
Without the retention plunger, the magazine cap or extension will tend to unscrew under recoil. It's the plunger in the support ring that retains the magazine cap or the extension collar.
Most Express barrels will not have the plunger. Wingmaster and Police barrels do.
You can buy a rifled barrel with the retention plunger direct from Remington.