Howdy Mike! Can't help ya with the 38 or 45, but I've been shooting 44 Colt loaded with BP for 4-5 yrs now. Fire em in Uberti Open Top and Richards II. Love the guns and caliber. I had the same questions when I started with the caliber-what powder, how much, what lube, bullets. Got answered by reading this web (CAS), reading magazines, books, and input from a long time shooting pard. Check out some of Fox Creek Kids, Mako's, Bottom Dealin Mikes, and others posts. Also Bot Delin' Mikes mag articles in Guns of the Old West.
Bullets---I stated out castin and loading a Lee 200 grain (still use it), then switched over to a Mav Dutchman big lube (200 g). For powder--- I always used Goex FFF in my cap and balls and used it also in my 44 loads. Fox Creek and other wrote of the advantages of using FF. Less peak pressure, slower burning, easier on the gun, etc. I use Goex FF for all my cartridge loading now. Other brands will work to, but they for the most part are more $$. Primers--the rule I first heard was always use mag primers for a hotter ignition. Read a number of posts and mag articles that regular ones did the trick. Use mags some, but have had excellant results from standard Winchestor large primers. Lube--- I've used Dicks Pearl lube and SPG both with equal and excellant results, just stay away from the petroleum based ones. As to how much powder, I've seen posts and articles all the way from 22 grains, up to 28. Most of what I saw from long time shooters/loaders was in the 25 grain range.
I played around with loading 25 rds of 44 Colt using a variety of components ( I love to load em as much as I do shootin, no chore thar for me!). Loaded each 25 rds with both Lee and Mav Big Lubes, used both CCI Mag primers and Winchest large pistol, and used both Goex FF and FFF powder. Loaded each batch starting at I believe it was 24 grains, then added a 1/2 grain (24.5), then 25.0, 25.5, 26.0, etc up to 28.0 grains. A shooting pard asked me if I thought a 1/2 grain would make all that difference, I said probably not, but did it anyway-(can be if your loading smokless). I found and have settled on a load of 25.0 grains of Goex FF (a load that Fox Creek Kid had mentioned in one of his posts). I use either the CCI or Win large primers, using either of the bullets I previously mentioned. Still use the CCI's just to use them up, saw no advantage to them. Note- I don't have or use anything to record and measure fps, pressure (a/k/a a chronograph), etc, just good ol' frontier results.
With the 25.0 you have the compression Sir Charles mentioned, good accuracy for short range and beyond, it's not a marshmellow load, nor a 'big boomer'. Always figured those that load for 'shock an awe' have forgotten that the purpose of shooting is to hit what your aiming at with a chunk of lead, not see how much smoke and fire ya can produce. Don't get me wrong, I love the 'smell of black powder smoke', the boom, and all that goes with it, but if all I'm tryin' to do is intimidate and impress others with 'shock and awe', I'll just go to shooting a cannon. Hope this helps ya Mike--Good shooting! Yers, Crow Choker