First off, I'm not a shot gun person, but I've acquired a 1995 Remington 870 Express Magnum which will take either 2 3/4" or 3" shells. I'm interested in installing a shorter barrel (20 - 22") for a house gun. With all of the variety of 870s made over the years are all 3" capable 870 barrels interchangeable and would therefore fit this shotgun?

Yes as will all or most 2 3/4" chambered barrels, just can't shoot the 3" shell in the 2 3/4" chamber.

Only difference may be the Light Contour barrels, dunno if they will fit but they might.

Also unsure if the Super Mag barrels will fit. If they do you just wouldn't be able to shoot the 3 1/2" shells on the regular Express Magnum receiver.

I will agree with 10guagemag. With your Express Magnum action any of the 2 3/4" or 3" 870 barrels should work. I have ran the 3 1/2" chamber barrels on an 870 3" Magnum receiver as well. With the 2 3/4" barrel you are limited to 2 3/4" shells, and with the 3 1/2" chamber you are still limited to 3" shells due to your receiver.

The exceptions are the 870 Special Field barrels and the 870 Tactical Home Defense model barrels. They won't fit on a standard 870 shotgun due to having a different magazine tube length. Also watch for if the replacement barrel you choose has a ball detent set up on the magazine ring the same as what you already have. Or if it will work OK with what you have anyway. Sometimes if the ball detent is not present on the replacement barrel you need a wave washer to take its place. No big deal.

Measure from the receiver end of the barrel to the ring on the barrel that goes over the mag tube. Just make sure any replacement barrel has the same measurement.

The difference between a 2 3/4 12 gauge receiver and a 3 inch receiver is the position of the ejector. The barrels will fit each other but not eject 3 inch hulls in the 2 3/4 receiver. A repaired ejector in a short chambered 870 is probably a 3 inch. The receiver is identical except for the ejector and a 2 3/4 inch receiver can be changed out by a good gunsmith anytime. They are riveted in place and usually need a re-blue after. You can shoot a 2 3/4 barrel in your receiver with no issues.

12ga 870s are easy, now 20ga 870s can get complicated