Andy, you're quite right that it's a bit like comparing apples to oranges. Yet the makers of Trail Boss seem to be promoting this handgun caliber powder as some form of universal solution when it's clearly not a great option for rifle size volumes if we want to avoid high chamber pressures on older guns or older designs.
For example, it's listed on the Hodgdon web site as a powder for use in "trapdoor" .47-70 Government loads. But the pressure it produces for the velocity is very high compared to the other choices.
And yes, if Bullseye, Tightgroup or Unique were listed in there I don't doubt that the pressure from those handgun powders would be up there too. But they aren't listed because they are not a good matchup. Yet TB is listed. And it produces the expected lower velocity from the expected high peak chamber pressure. Which, to me, makes it a bad choice.
Jason, like you I've found that the reduced velocity loads using the powders you listed do produce some unburned powder flakes in the bore. But not so much with 4227 and 4198. But the amount they leave is minor compared to the amount left by5744. I've yet to try the 3031 I got some time back. But the nice thing is that all of these powders run at a lower chamber pressure which I like due to shooting from an 1869 vintage Swedish rolling block action.
I just started trying some loads that use a tuft of polyester pillow stuffing to hold the powder back against the primer. The hope being that I'll get a more complete burn. But really as long as the burn is consistent does it really matter if there are a few flakes left in the bore? It's not hard or abrasive so it's not doing any damage. At least that's what a few long time shooters have been telling me. I'd LIKE the charge to burn fully and cleanly but in the end tight groups is the goal. If I get that with some powder flakes left in the bore then "meh.....". I know it runs counter to our desire for tidiness but we just need to deal with that...
At any rate it's hard to find load data with pressure for .577 Snider. But based on the data I see for things like .45-70 it would not be one I'd want to use in an old action. And once again looking at the load data at the Hodgdon web site it's interesting that .50-140 Sharps does not list TB as an option. And the data for 45-120 Sharps does list it but at the maximum load with a pressure that is close to the other maximums the velocity is only about 2/3's that obtained for the same sort of pressure using other powders.
All of which suggests to me that TB is a powder best used in handgun cartridges or at least in smaller volume rifle cartridges.
My old time rifle cartridge of choice is .38-55. And even with the skinny 55gn case TB is a poor choice based on this pressure vs muzzle velocity issue.
Look the data over with an eye to the pressure vs velocity and I think you'll agree.