Greetings backrub21, and welcome aboard.
You can use shells that are shorter than the max allowed by the barrel. Confusion occurs when folks put a 2-3/4 to 3" extra barrel on an old 2-3/4" action. Just because the barrel will accept the longer shell, the action won't. That's what the Remington note, that TheKlawMan referenced, is saying. It seems there are some dimwits who thought that by putting a new barrel on an old 870 would some how magically turn a 2-3/4" gun into a 3" magnum. With your supermag, you can shoot anything you want.