Doc,
I have watched for a while with interest hoping someone would answer who is actually shooting pistol or shotgun powders in .45-70 loads. You will often get as many answers as there are days in the month. The info on GMDR looks like it might be good BUT, when I see loads for 300 gr bullets with
Tight Group, Bullseye and Tight Wad I get a bit ummm (call it concerned).
I can tell you what I had to do a few years ago to continue to load large volumes of pistol cartridges that used Unique, HP-38/W231, BE-86 and CFE Pistol. I finally found two large containers of one powder and was able to find loads for lead bullets in 9mm, .40 S&W, .45ACP, .38 Spl and .44 Spl. that could all use that powder. I laughingly called it Winchester "Unique" because it worked in so many for "heavier" lead bullets in all of those calibers.
The secret is to find a powder that has a similar burn rate and a similar density. That will give you a similar volume which is a component of the pressure the load will generate in a cartridge with a fixed volume. The problem I have run into is that the charts from different sources do not always agree, even the charts from companies like Hodgdon don't always agree from year to year. But, you can look for a trend in agreement.
Look at the portions of four charts below:
You will notice that N340, WSF and Herco seem to switch back and forth. Now look at this one:
I simply show that to you because it has some more European powders on it and more importantly you see the powders ranked by manufacturer.
Now look at the comparison of the six powders below that chart based on the Volume Measured Density. You will see that Universal, N340 and even Herco are similar in density while Power Pistol and Win WSF are denser. And, once again depending on the chart you believe WSF appears to fall in the same range as Unique while on others Power Pistol is faster. I can tell you that WSF is a slower powder, but since it is denser you can load it in a small volume case like a 9mm and make your velocities and keep the pressures lower even though that last chart makes it look as fast as Unique or Universal. I can also tell you Power pistol is relatively "hot" and probably a poor substitute for Unique. For instance you can't find any shotshell load data for it anywhere.
The shotshell data is another clue to a substitute for Unique or a heavy bullet load (which 300gr is relatively speaking). If you can load 1-1/8 or 1-1/4 ounces of shot over a load of powder then you probably have a powder that will perform with a heavy bullet mid-range or low velocity load. Unique does that and so do the other 5 powders (other than PP) I show.
Where your application is "unique" (no pun intended) is that you are using it in a high volume case (.45-70). Personally (I'm a bit spooky when it comes to low volume powders in large cases) I would stay away from the faster traditional shotgun powders, all of the "dot" powders, the clays, etc. except for Universal which used to be called Universal Clays. That is a very versatile powder like Unique. WSF is also a "shotgun powder" but slower in the Winchester family.
I usually use Trail Boss for reduced loads in big cases like .45-70, but I understand it has gotten expensive and very hard to find. I'm lucky happen to have a big container of it I may never use up. I shoot a 405gr with BP or Trail Boss.
But when reduced to hunting for ANY powder to shoot, look for similarities like volume and burn rates. the charts above will give you starting points. Perhaps some of the boys who shoot over at the Plum Creek Shooting Society (I liked it better when it was called the Plum Creek Carriage and Shooting Society, it sort of gave it an air of gentlemanliness) will see your Post. Ask Abilene if he knows of any cowboys (or cowgirls) over there who shoot those types of loads, He may not now but he used to attend their matches.
~Mako