44-40, also known as 44WCF (44 Winchester Central Fire) has a slight taper to it and is almost as wide at the rear as a 45 Colt. 44-40 will not fit in a 44 Sp or 44 Mag chamber. 44 Mag or Special will rattle around in the chamber. 44-40 guns will only fire 44-40, nothing else.
Here is a photo of some 44 caliber rounds, left to right they are 44 Henry Rimfire, 44 S&W American, 44 Russian, 44 Colt, 44 Special, 44 Magnum, and 44-40. If you look very carefully you can see the slight taper behind the 44-40 bullet.
A 44-40 Ruger is not rare, but not all that common either. That is probably a good price these days, but you can only fire 44-40 in it.
P.S. 44-40 used to be almost obsolete and difficult to find but Cowboy Action Shooting has revived interest in it. Today you should be able to find 44-40 Cowboy ammo almost anyplace you can find 45 Colt Cowboy ammo.