Off the top of my head I can’t recall which powders are not position sensitive, Dave. Unique usually isn’t but it’s not a good choice here. 5744 doesn’t seem to be position sensitive and it’s generally a good choice for BP cartridges. I don’t think 3031 or H4895/IMR4895 are but I don’t trust my memory. Most good loading manuals will address this trait if applicable, 

  In Williamson’s book on page 13 he states “My personal documented experiences include firing 750,000 rounds and maintaining a journal of the firings. The journal not only records the firearms and bullets used, but also includes the targets, recovered bullets, pictures and autopsy notes, indexed and categorized for study”. Pages 81 thru 99 cover the 45-90 data including 3031 and 4198 powders, bullets, brass, groups, vel., the most detailed information I have seen. Page 617 addresses powder-fillers. I looked on Amazon and used books sell for $55. It is a amazing reference. T/R

I really enjoyed Snooky Williamson’s book as well. Only criticism I’ve heard are that his loads are a bit warm but the ones I’m familiar with are reasonable for a rifle in very good working condition. As always, back off a bit and work up.

I don’t think IMR4895 would be a bad choice for the 45-90, Waters lists loads for IMR4895 and H4895. Pretty sure it will fill the case well enough that position sensitivity wouldn’t be an issue but I haven’t used it in 45-90.

  I went thru a couple dozen original Winchester rounds in 45-70, 45-90. 40-82, 40-70 that had jacketed bullets with smokeless era head stamps and they all rattle when you shake them, I would guess that means no filler.T/R

I experimented with smokeless 45-90 loads a number of years ago in an original ’86 made in 1888. The main problem with the 45-90 case and smokeless powder is that there is an awful lot of empty space to fill. As others have pointed out, 5744 is less position sensitive that many other powders. I found that IMR 4227 was frighteningly position sensitive, with huge choreographed differences depending upon whether the powder was fore or aft. So was IMR 3031. To solve this problem, I used a single sheet of single-ply toilet paper loosely rolled and folded. However, I perfectly understand if you do not want to go there. IMR 4198 is another powder that seems to do a bit better when it comes to position sensitivity and consistent burning. I certainly would not worry about copper jacketed bullets. Copper is considerably softer that even mild steel.

I also found that oversize bullets tumbled in a truly astounding way (.002 oversize to the groove diameter). I have no idea why this was the case. The best results were obtained with .457 soft-cast (air cooled clip on wheel weights) bullets, but it seems you have different results.