The Marlin would handle hotter loads than factory, but can't
be opened up in the port for real long cases. The Mossy 395,
495 and 695 can go to 3.5" brass cases, and looks nicer.
The loads I do with the RE17 could be used in Marlin 3 inch,
that I tested above in the Mauser with regular slug barrel.

Here is a novel slug idea, hard plastic support ring on
the front, and a locked base ring/seal. Metal never
touches the bores. Work in great rifled and good in
smooth barrels. These are steel, lead/copper would do
great also.Could be made heavier and different points.
Flat point shown would do great damage to game.
On the net- ammunitiontogo.com
This one is about 500 gr.

Second picture is the 499HE to use in redoing the 500S&W,
shown on the also, in single shots, etc,for those wanting
much more power at 40-50% lower pressures.Ed