577/450 -
It appears from your original post that your interest may be in trying to get a lighter load for shooting in your carbine-length Snider - if so, of course nobody really sells commercially loaded .577 ammunition, and the original rounds are too valuable as collector cartridges to consider shooting them off - if, indeed, they'd even be reliable .....
In the original military loadings, there never was a reduced "carbine" load for the .577 Snider. The chambers of the carbines were precisely the same as in the rifles, and thus the full-length rounds fit in them. Carbine users were apparently expected to just "suck it up" and shoot with the standard cartridge. (On the other hand, as you may be aware, the British found that they had no choice but to develop a "carbine load" for the even harder-kicking .577-450 Martini-Henry stock design, and in fact the actual issue of M-H carbines was delayed for more than a year after the carbine patterns had been approved, so the carbine load could be developed and put into production.)
Accordingly, if you simply want representative cartridges for display with your Sniders, the full-length cartridge is "correct" for all models. You could, of course, pick up an example of the shorter .57 Snider commercial loading to show what that sporting cartridge looked like.
On the other hand, if you want ammunition to shoot, that is pretty much a home reloading proposition. As an essentially straight-walled cartridge, the .577 Snider can be easily "downloaded" by simply reducing the charge of black powder - to say only 50 to 60 grains - and filling up the space between the top of the powder charge and the base of the bullet with a simple filler like corn meal or cream of wheat granules (..... dry and uncooked, of course! Believe it or not, someone not so long ago got this information from the internet and actually cooked the cereal up into mush .... then used it as a filler!)
As may have already been mentioned in this thread, if the shorter .57 Snider cartridges were consistently fired in any Snider (carbine or rifle) before it was "retired", fouling build-up in the area between the case-mouth and the throat of the chamber may have effectively reduced the length of the chamber, so that a normal-length cartridge will no longer chamber properly until the obstructing material is properly removed.