A gunsmith familiar with the Spencer action needs to chime in on this ,,, but, as I understand how the cartridge feeding works, seems to me it could probably be done
if a new breechblock made for that shorter length of cartridge were made and installed …
and if the length of the butt was increased a bit to accomodate a longer magazine tube (which in turn would require a lenger follower spring and tube …)
Dealing with the latter problem first - Spencer cartridges were/are in the range of 1.60" OAL (which, I understand, is why the action can be used more or less "as is" to make repros chambered for, say,.45 Colt - which has pretty much the same OAL.) If I'm not mistaken, the .45 Cowboy special has an OAL of about 1.25" … so you gain about 0.35" of "room" with each shorter cartidge. Accordingly, over the magazine capacity of seven 1.6" cartridges … you only gain 2.45" of room (0.35" x 7) which is not quite the combined length of two .45CS cartridges …. so a standard-length Spencer mag would only hold nine cartridges of .45CS length. However, if the butt stock and magazine were lengthened by, 1.3" or so (… wouldn't be too much of a problem for most "modern sized" people, would it? …) then you could have a magazine long enough to hold ten .45CS cartridges ….
The other problem, however, would be the geometrics of the action itself ...
AS I understand it, cartridge OAL is fairly critical in a Spencer because, as you depress the lever, the next cartridge is pushed forward into the "cut-out" in the big breechblock (area marked above by the two parallel red lines) … then, as you close the lever, the breechblock rolls forward to start the cartridge feeding toward the chamber … presumably with magazine spring pressure still being applied to its base by the nose of the following cartridge ... until the edge of the block cutout (marked 'X') moves into position to catch the rim of the cartridge and continue pushing it forward, while the curved surface of the breechblock rolls up in front of the bullet of the following cartridge, preventing any further forward movement by it (… and, if anything, probably pushing it back into the magazine tube a bit.) If the geometrics of the block cutout were changed (along the other V-shaped red line) I presume the Spencer action could be made to chamber cartridges of shorter lengths …
Indeed. I wonder if the flat face of the cutout in an existing breechblock could simply be "faced" to work with shorter cartridges …
(Don't imagine you could adapt the Spencer action to work with cartridges which were much longer, however, because the size and bulk of that already big breechblock would have to be increased …)