One fellow came up with a scheme to create a "controlled feed". He's also come up with some mods to allow VERY SHORT cartridges such as the Cowboy .45 Special to run in the marlin without a handmade, dedicated carrier and other extensive mods.
As a "build along", I've been doing these mods and some additional tinkering with a .44 mag 1894. Though the carrier in the gun now has been through several versions, and is now patched with JB weld, etc, an hour ago I loaded three .44 mag, 240 rnfp loaded to a 1.560 OAL, three .44 special 240 SWCs, and three .44 Russian 200 rnfps at something like 1.255 OAL, alternating them, and they ALL fed. Some fed better than others (the SWC .44 specials fed best), but I'm tantalizingly close to a true multi-cartridge Marlin

Waddaya think?