Years ago, Dixie Gun Works made turned brass cases for the rimfire spencer. You put a 22 blank in an off-centered hole in the base. The edge of the blank rim was in just the right place on the rim for the firing bar to hit and set of the charge. The held a reduced load of BP and regular bullet, but allowed you to at least shoot them single shot.
Seems to me, you could do the same thing with the .46 rimfire! Make some chamber casts, turn a brass case i9nside and out, then mill out the offset hole in the rear face of the rim for the blank. The should stay in the right alignment in the cylinder, if you put them in right.
I don't own an original, but I know I love to shoot my original 1860 Spencer .56-56 with its S&S centerfire breech block
I tried using the Dixie brass in a Spencer and very seldom could get then to fire. I had to fumble arround trying to get the primer in position for the firing pin to hit it. It was more difficult than I thought it would be. I finally gave up.
I tried using the .41's in a Remington deringer and had no luck getting them to fire,perhaps the firing pin was worn.
I have used the .36's in a Colt 1851 Navy conversion and they worked OK, but the balls had to be seated very well or they would
come loose.
Tubac
I have an original 1860 SPencer Carbine in .56-56. I use a S&S centerfire conversion breech block and modified and shortened 50-70 brass from Buffalo Arms with a 375 grain (.435 diameter) Rapine heeled bullet. I load 39 grains (volume) Hodgdon's 777, CCI Magnum rifle, and paint Lee Alox on the outside. I use the CH 4-D die set specifically made for 56-56 Healed bullet loading. It was easier to use the centerfir breech block than screw around with the rimfire rounds from Dixie.
A paard of mine has an original Remington double derringer, and shoots the Dxie ammo. Works good, but hotter than older original loads he has tried.
They make one for .44 rimfire, too. So original Open Top owners take note.