Remington has made so many 870 shotguns, over 8 million at present, and millions of spare barrels, that it is hard to give a definitive answer without inspecting the barrel. A lot of the barrels that will take 3 inch shells, also accept 2 3/4 inch shells. They will have printed on the barrel, "2 3/4 inch shells, or 3 inch shells in magnum receivers." If the barrel is marked, "only for 3 inch shells and magnum receivers," it may not work on a 2 3/4 inch receiver. That's why it's necessary to inspect the barrel.
Even if the 3 inch chambered barrel "fit" on a 2 3/4 inch receiver, it would still only work best with 2 3/4 inch shells. The "standard" receiver is shorter, and set up with a stroke distance, and ejection port opening for the shorter shells. You could load the 3 inch shells one at a time through the ejection port opening, then close the bolt and fire the gun. But the longer, 3 inch shells may or may not properly load out of the magazine tube, and may or may not properly eject out of the gun when you pumped the gun and tried to eject the longer shell out of the shorter receiver.
If you are trying to make a "magnum" gun out of your "standard" 2 3/4 inch gun by putting on a 3 inch chambered barrel, you're working against all the design engineers and gunsmiths that have been working on the 870 since its first year of production in 1950. I happen to own a first year production 870AP, made in April 1950. My dad bought it then, hunted with it for 9 years, then gave it to me in 1959 on my 10th birthday. So this gun has been in my family for almost 55 years and I've been hunting with it for over 44 years. A couple years ago I decided I wanted to hunt ducks. I didn't want to shoot steel shot out of the original 26 inch modified choked barrel fearing I might damage it, so I found a 28 inch Express barrel with RemChokes. It was printed on the barrel, "2 3/4 inch shells, or 3 inch shells in magnum receivers." I've been using it on the 1950 870AP for ducks, shooting steel shells with a modified choke tube, but only using 2 3/4 inch shells. It works fine. I would not attempt to shoot a 3 inch shell in it because I know the receiver is the shorter "standard" model and not designed for 3 inch shells.
TennSquire
