I wouldn't think of using a smokless powder load in a Spencer rifle or carbine.   Wondering if anyone has ever tried smokless powder loads for the 56-50 in sturdier rifles.  Case in point is my Peabody 56-60 RF carbine that has been converted to center fire.  Fun rifle to shoot with black powder, but sometimes I feel a little lazy.

George

I think the 56-50 cartridge is a bit shorter than the 50 carbine used in the USN rolling block.  I've not used any Blackthorn 209.  Years ago I tried pyrodex in several other rifles and after that swore off black powder substitutes.   I use either black or smokless.   Thinking I might try some 4759, or perhaps 4227, for starters.   Just trying to figure out a safe charge;  maybe about 55% of something shown for the 50-70.

George

Is your "Spencer an original or a replica?   I had an original 1860 Spencer that had been reworked into an 1865, presumably be Spencer or Springfield,  and it needs different brass than the Peabody.  The Spencer brass will chamber in the Peabody, but not the other way around.   I can use reworked 50-70 in the Peabody and an original 1873 FN Spencer made in centerfire, but not in the US made Spencer.   The 50-70 rims have to be turned down so that they will go into the magazine.  I've had some problems with feeding, but usually it seems that the overall length is too long.  The FN spencer (one of less than 1000 made originally in centerfire) does use a slightly longer case than the  US Spencer and Peabody, but not as long as the one you show.   Maybe a little less than 0.10" or so.

Playing some number games and coming up with a starting point of about 14 grains of either 4759 or 4227.

I'll have a look at that Spencer site.

George

I'm not saying this is the same for the .56-50 Spencer, but offer it as an analogy.  The late John Kort showed that for .44 WCF, a full case of Reloader 7, about 25gr and IDeal 427098 bullet was equal to the original black powder round pressures and velocity.  

With that in mind, I followed the same philosophy when working up a .44 cal load for my #2 rolling block converted from rimfire to centerfire.  This .44 Remington centerfire case is .965" long and made from .41 magnum brass.I ended up using 16.8gr of RL7 under a 208gr bullet. This was a full case load under the heel bullet. This gave an average velocity of 1000 fps. This was noticeably milder than my stoutest load of 23gr of Old Eynsford 3F that gave an average of 1124fps.  

Based on John Kort's research and experiments, and my own experiments, I'm comfortable using up to a full case load of RL7 in pistol size cases.  I've used quite a lot of it in many small vintage rounds.  I think it is an excellent smokeless choice under the right circumstances.

Your mileage may vary, I only offer this to share my experiences.