Maybe it's a regional thing, but hardly anyone I know shoots 44-40.
Some do, and you will find other black powder cartridges as well.
This is overly simplified.
Most of the people I shoot with use either the .45 Colt or .38 Spec.
The .45 Colt is the "cowboy" caliber and is used by those trying to stay true to the cowboy way, although Wild Bill used .36 Navy.
The .38 Spec is used instead of the .38 S&W that was popular right at the turn of the century. Some people will use the .38 in their revolvers and .357 in their rifles, most notably, the Marlin, as it does feed somewhat more reliably. I use .38 in both and had a .38 carrier installed in the rifle. The only problems I have are operator induced. (Don't ask)
Charlie left out another group who shoot the .38. I'm cheap and reloading is relatively inexpensive.
The .32 Magnum is used by those wanting the least amount of recoil to get back on target the quickest. Cowboy shooting is a timed competiton.
These are the most common, but in no way limited to these.
The SASSNET web site will certainly give you divergent opinions on almost anything, and a few will be knowledgeable. The Fireing Line has a Cowboy/Black Powder section. While good, it doesn't get used much.
A few moths ago, there was a good basic article in American Rifleman plus some very basic information on reloading.
Check around those web sites, hear what they have to say, use a little common sense, then come back here. Charlie, among others, have been most helpful and helped me put it together.(Considering the challenge in that, and I'm not east to teach, I'm happy with where I'm at.)