You are probably familiar with how Remington 870 shotguns are made with different gauges. The most common gauge in the model 870 is the 12-gauge. This measures the diameter of the barrel. But what you may not realize is the length of the shells is also as critical to consider as their diameter. If you have 3 inches or 2 ¾ inches shells then you need a magnum receiver that can handle it. Of course, the barrel also has to be equipped to handle up to 3-inch long shells. If it is then you can fire them as long as you have a magnum receiver.
Video which shows drilling and tapping of the Remington 870 receiver to install.
Thanks to Andrew and Bill for help and comments. Thanks to S&J Hardware for the rail.