low PSI smokeless loads for the 50-70 government.

I have never reloaded for a 50-70 Trapdoor but I do load regularly for 1873 and 1884 45-70 Trapdoors. I use 28 gns of H4198 with a 405 gn lead bullet. This gives a mild load that produces about 1250 FPS which approximates the original black powder load. According to Hodgdon the pressure for this load in 45-70 should be about 15,000 CUP. I would advise against using Unique as it would be easy to double charge which would be disastrous. Also would not use Trailboss as it produces quite high pressure spikes for the velocity it provides. You also might contact Hodgdon for their recommendation.

Lyman recommends keeping Trapdoor pressures under 18,000 CUP.
The max pressure for the 50-70 is 22,500 PSI. (different than CUP)

troy fairweather said:

can't get BP in NY i have to go to PA and pyrodex is over $25 a pound, i think i remember dad using 5744,

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I'm in PA and AA5744 costs $32 and more. It seems to be very expensive so if you can get Pyrodex for $25 # I would go with it.

According to my 2004 Accurate Arms reloading manual

Powder:AA5744
Barrel: C. Sharps
Barrel Length 30"
Case: DGW
Primer Fed 215

(L) 425 gr SP COL 2.275"

Start 27.0 grains 1305 fps, max 30.0 grains 1419 fps

(L) 550 FN COL 2.200"

Start 22.5grs 1111 fps, max 25.0 grains 1208 fps.

Note, reloading manual did not provide pressures and used a modern C. Sharps barrel, and I assume, a C.Sharps action. However I have used the low pressure AA5744 data for the 45/70 in an original 45/70 trapdoor and did not have any issues. Accurate Arms claimed they developed AA5744 for those big blackpowder cases. I suspect they found an existing powder and determined it would work well in this application. But, of the powders I tried with cast bullets, AA5744 has performed the best in 45/70, and in 30-06, 303 Brit, 30-40 Krag.

I have some amount of data and experience loading cast in .45-70 for my Pedersoli 1885, as a rule I try to keep my pressure below 15K PSI. AA5744 would probably be the #1 pick, it was designed for the .45-70 (and other big boomers) as a bulky powder similar to black... without all the mess. I am a BIG fan of IMR4198, it works very well in the .45-70, and I would think the .50-70 as well. Although I don't have specific data for the .50, you could probably go the other direction with 2400 or Unique, but the 'faster' powders peak on pressure pretty quickly... but they are good for reasonable velocity (1100-1300fps) out of a rifle barrel. I think the bulky Trail Boss is another 'pistol' powder that would be good for reduced loads, too.