Evan, The above posters are correct, the 44-40 has a slight bottleneck, is actually slightly longer(1.305 vs 1.285) and has a larger body diameter near the case head(.469 vs .457). So no they won't interchange worth beans.
I imagine one could try to size and trim 44-40 brass down to 44 size, yes(even then, the 44-40case head diameter is about 10 thousandths larger, so it may not have room in your revolver) . BUT-
1. 44-40 brass is notoriously thin/weak near the case mouth.
2. You would end up with brass that was mis-headstamped
3. I doubt that 44-40 brass is strong enough in the web/head area for 44 magnum pressures, as it was NEVER designed as anything approaching a high pressure round.
Eric F.- Nominal is .427, but bores on the high side of .430 have been reported. So if you have one, slug your barrel.