The Marine Mag is the same grade gun as the Police Magnum, 'cept the finish, furniture and markings.
What you refer to as a "regular 870" is probably the economy, Express model with a bead blasted blued finish that could develop surface rusts if you don't pay attention. Not to mention other economy related issues and a lesser QC.
To get back on topic here is a write up I pulled off another site on the difference between the MM and the 870P
The Express is Remington's "budget" gun, made to compete with the cheaper to make Winchester and Mossberg guns.
The Wingmaster is Remington’s "Cadillac" top-of-the-line sporting gun.
The Police is a Wingmaster with a dull finish and is a much more carefully inspected and built gun.
The new "Tactical" and Marine Magnum guns are based on the Express.
How Remington lowered the Express price was to reduce hand labor to a bare minimum, and to eliminate much of the polishing and de-burring the better quality Wingmaster and Police guns get.
The Express is basically the same forged and milled steel receiver and heavy-duty internals gun the better 870's are, just in a rougher, less well finished form with plastic and MIM parts.
The Express Model has:
A plastic trigger group.
The dimples in the mag tube and the new style plastic magazine retention system, EXCEPT on the extended magazine versions, which do NOT have the dimples.
A rougher finish inside and outside, with machine marks and some burrs left.
A rougher, bead blasted blue job.
A less polished bore.
A two piece sleeved barrel. (not 100% sure about this)
Hardwood or synthetic stock, with a sporting-length fore end and pressed-in checkering.
The Defense version has 18", Cylinder bore barrel, with a bead sight.
The Marine Magnum and Tactical models have plated or polymer finishes.
Some Metal Injection Molded (MIM) parts, like the extractor.
The Wingmaster has:
An aluminum trigger group.
The old style magazine retention system.
A much smoother finish inside and out, no machine marks or burrs.
The Wingmaster gun receives a higher level of inspection and finishing.
A fine, commercial polished blue finish.
A polished bore.
A chrome plated bolt.
A one piece barrel.
Walnut stocks with the famous "Bowling Pin" finish in gloss or satin. and better checkering.
Wide choices in barrel lengths and choke options.
No use of MIM parts, the extractor is milled.
The Wingmaster is the full top-of-the-line commercial Remington pump gun, and is priced accordingly.
The 870 Police has:
An aluminum trigger group.
The old style magazine retention system.
A much smoother finish inside and out, no machine marks or burrs.
The Police gun receives a higher level of inspection and finishing and is built in a special area of the plant from 100% inspected parts.
A military-grade parkerized finish.
A polished bore.
A one piece barrel.
Walnut or synthetic stock, with a short police-length fore end.
The Remington “R3” super recoil pad that reduces felt recoil by 30%.
Choices in different stocks, including Speed-feed, and others.
18" to 20" improved cylinder barrel, with a wide choice in sights, including rifle, ghost ring, and luminous.
Police options like magazine extenders, forearms with built-in lights, and sling swivels.
Heavy-duty magazine spring.
Heavy-duty trigger-sear spring.
Heavy-duty shell lifter spring.
Sling swivel mounts.
No use of MIM parts, the extractor is milled.
The Express is a "bottom of the line" budget gun, the Wingmaster is a "top of the line" sporting gun, the Police is the top-of-the-line in defense guns.
