Originally Posted by hookeye

Nope.

The 3.5 magnum and reg 3" magnum have the same mag tube length.

Per Remington's owner's manual............you can put three 3.5" shells in the mag.
You can put four 2 3/4" or 3" shells in the same mag.

The receivers are even the same length. The ejection ports of an 870 Supermag are a little bigger (and of course there are some parts that differ internally).

If you buy a 3.5" Supermag you can run it as a 2 3/4" or 3" gun.

Even the barrels interchange (within standard models- some home defense stuff has the barrel ring farther out/magtube of different length).

You can even put a 3.5" bbl on a non 3.5" receiver. But the warning label on the bbl tells you to run 2 3/4" or 3" ONLY.