ive been using trail boss for over a year now and i think i can post a finalized report of my expiereince with it.
firstly, many of the book loads are VERY conservative IMO. i was able to more than double the reccomended max load in the 45-70 with no pressure signs whatsoever, and was at 150% in the 38 spl. i DO NOT reccomend exceeding published max loads unless you are very sure of what you are doing. in this case however i think they WAY undershot the potential of the powder in modern guns. in century old originals yeah that might be a good limit. But for modern rifles you can work your way much higher than thier published spec.
ive loaded everything from 9mm to 45-70 to 12 ga shotshells in testing it, and have come to some conclusions. in the application of cast bullets in straight walled cases like 38 spl, 357 mag, 45-70 etc it simply cant be beat in the categories of powder cost per round, ease of use and consistency. When you move outside that comfort zone its usefullness is limited.
The powders design makes it inherently bulky (the powder looks like mini cheerios, has a hole in its center) and only 9 oz comes in a powder container that normally hold 1 lb. I can attest it goes a LONG WAYS when you are loading. i use it exclusivly now in my 45-70 500 gr BPCR silhouette load, and use it for 38 and 357 mag lead plinking loads. its recoil is minimal compared ot other flake powders and its bulky nature makes it impossible to double charge a case. it IS NOT a powder for progessive relaoders IMO. about 5-6% of the time the powder hangs up in the drop tube and requires a tap to fall. with a seperate powder measure this isnt aproblem, but with progressives can be a major issue.
it fails miserably as a 9mm powder simply because there isnt enough case capacity. i only got about 500 FPS with a 124 gr cast bullet and it failed to cyclethe slide every single round, evne when i filled the case to the mouth and compressed the load. i had similar results with 40 S&W and 10mm auto. this powder simply doesnt work with small, rimless pistol rounds. i loaded a small batch of 45lc for a friend with it and he was happy with them, but only shoots it occasionally as its his belt gun for around the farm. he says its clean, doest kick much and works well.
it alos is poor for shotshells. i was thinking thios would be a good powder for brass cases with felt and waxed card wads, but it didnt really do anything that red dot or unique didnt do with a smaller load. if thats all you have, yeah TB will work in a shotshell, but there are far more economical choices.
thats about it i guess. If you use it inside the wondow where its intended to be used, you will be very happy with it. move outside that, and you may be dissapointed in its performance.