The .44-40 base diameter is .471, the neck diameter is .443, so the case is certainly NOT straight; those cases are tapered in such a way that a real shoulder is not evident. Many .44-40 cases today are made that way and really present no problem. The "shoulder" may appear after they are fired, but I recommend checking the sizing die to be sure it has the shoulder. If not, that area is swelled and squeezed over and over and will fail rather quickly.
FWIW, the statement that a "straight .44-40 would be a .45 Colt" isn't true. They are different cartridges with different base and rim diameters. The .44-40 came first, designed by Winchester for its 1873 rifle. Colt developed the .45 Colt because the Army wanted a .45 caliber, but had to cut the rim diameter down to fit six rounds in the cylinder. That small rim gave extraction problems in rifles and revolvers that used an extractor (the SAA Colt, of course, had a rod ejector), and until recently, rifles were rare in the .45 Colt caliber. Now that the "cowboy" craze has struck again, there are rifles in .45 Colt, and (surprise!) they still give trouble due to the small rim.
Jim