Deacon,
Have you ever annealed cases?  I have annealed the mouths of my my .38s and .44 Russians to help with the obturation upon firing and to allow me to put hard crimps on the bullets without getting cracks on the mouth.  I have not performed designed experiments to determine how well this works for straight wall pistol cases but i have done it for some time for my .308, .300 blackout, .30-06 brass and even my better loads in 5.56.  I definitely need it on my match .308 and '06 loads for my semi-auto rifles.

The "problem" with heavier cases like the .45 Colt is getting adequate expansion at BP or BP substitute pressures.  .38 spl. cases are a bit thinner and I do it mainly because I have an auto annealer I built, and I can just load up the hopper. The .44 Spl feeds okay but the shorter .44 Russian need to have the case added one by one to the drum and a hundred cases eats up almost an hour with a 4 second anneal, the handloading loading is the time adder.

You will find .45 Colt brass runs all over the place.  Some as thin as .012" but John Boy was measuring some time back in an earlier discussion about the appropriateness of .45 Colt brass with BP and reported from about .019" to .024" inches of wall thickness at the mouth area down about 5/16ths" into the case on 12 different brands.  The .38s are much thinner running about .010"-.012", if you use .38 brass that has a straighter wall  made expressly for wadcutters you will get .010" for a depth of about .5".  You can tell those match cases by the cannelure about midway down the case.  I have a bunch because I used to build PPC revolvers and hoard the straight wall brass for long hollow base wad cutters.

This is all wadcutter brass, the 2nd, 4th and 5th are for hollow base wadcutters.



So, .38 is thinner walled and with well crimped loads (and heavier bullets) builds enough pressure to swell that case and seal the chamber as well as a Remington or Win .44-40 case in my experience. BUT, Starline makes a very thin walled (at the mouth) of about .0065" which seals exceptionally well.  They make that brass for people who own original .73s like mine which shoot the Ø.027" bullets better than Ø.028" bullets .  That thin walled brass works very well for Black Powder loads in modern reproductions.  Now don't get the idea that all of your sealing takes place at the mouth, the neck helps too.

I have been shooting TOP BRASS .44 Russian Cases and they are even thinner than my Starline .44 Russian and .44 Special cases.  they run around .010" to .011" at the mouth.  They were a bit hard and the annealing helped with that.  Starline Brass is always annealed when I get it and doesn't work harden much with BP load unless you have a an aggressive sizing die.  However a HARD crimp does work the brass and it needs to be annealed every 6 or so reloads if you don't want any splits.  I have some .44 spl brass I have been using for over 25 years and it looks almost new when I tumble polish it.  (I actually have match .308 brass I have had for over 40 years and only lose it or scrap it when it gets dented.)

~Mako

I will tell you this though, I get more residue back in the action from racking the action as fast as I can shoot than I ever do on the range taking my time between shots.  There are speckles all over my '66s from black powder still smoking as I eject shells during a match.  I get as little as I do with my original Win 73 in .44 WCF when I am taking my time between shots (no hot unburned  powder flying...)

~Mako

My 1860 Henry and 66 both wear that patina on the receiver.

Here is a picture of still burning powder coming out of my shot shells as they are ejected from my 1887

First question, is it 2F or 3F?

3F is for modern muzzle loaders only.
2F can be used in cartridges, shot shells and modern muzzle loaders.
Hodgdon does not recommend that any grade of 777 be used in antique firearms.

Load it to just shy of the base of the bullet. Hodgdon does not recommend compressing 777.

Link to Hodgdon's loading data for 777

https://hodgdonpowderco.com/wp-content/uploads/2023/04/cowboy-action1-1.pdf

My 92 is not as fussy on 38 Special cases as a previous one, so using a 158 grain bullet feeds well. I have never shot bunnyphart loads, so I would use a proper load. Considering trying the 38s as my BP set up as I just got a stainless Vaquero and have a pair of Conversions to handle the BP stuff.

Best BP rifles are any that take a necked cartridge, - .32-20, .38-40, .44-40.

But, your straight walled case then I would go with the .357.