AA3100 is a bit too slow burning even for BP velocities in a .45/70.
H4895 is a good powder for bulking in .45/70 cases with all bullet weights. It burns quite well down to about 60% of case capacity. In that 2.1" case, 3031 is another old time favourite, but it works better with closer to full-house loads and is suitable mostly for the stronger actions, not BP ballistics. H4895 is better suited for these.

In the first .458 2" long throat I built, which is very similar to the .45/70's 2.1" and .45/90's 2.4" case I experimented to a fairly large extent with almost all powders listed for the .45/70 - using cast and jacketed bullets. The faster powders as Dan observed were very poor choices as they produced erratic results in pressure. With the other powders, a full case always gave the best results whether I was looking for .45/70BP ballistics or modern high pressure loads - one must gain the knowledge to use the appropriate powder for the job at hand.

The rifle test-bed for these tests was a Mauser M98 action which proved it's value when things got out of hand, as in pressure excursions with some of the 'small rifle' & pistol powders listed in the books. IMR and Hodgdon's 4227 were about the worst powder for bouncing pressures. My unique test was very short as well.

For filler with reduced capacity loads, PufLon wasn't avilable back then, so I used Kapok and Dacron along with GREX by Winchester and later, some filler made or sold by Remington. Unlike the picture of the .45/70 case with a tiny puff of dacron on a tiny charge, I reduced the charge a couple grs. and filled the case above the powder with either Dacron or kapok to a compressed load. I discarded the use of Dacron as I preferred the kapok(from an old-style life preserver) The inert fillers today, ie: Grex and Rem's filler and Puff Lon are better yet. I don't particularly like the cerials that some use, as in COWheat or flour, as they compact into a plug, which can increase pressure by itself and being quite heavy, add to the projectile's weight, which also increases pressure.

A VERY heavy crimp is needed in straight sided cases, especially with slow powders to help promote proper powder burn. Too, standard primers, while igniting black powder well, do not do so with smokeless powders in straight sided cases. I used mostly Fed 215's with CCI250's and Winchester Large Rifle Mag primers filling in when I ran out of Fed 215's. This was years ago. I now use CCI250's and Fed 215's for all straight sided case loading. I've also tested with double primers, but that is in another thread and not needed for black powder. I've drifed away from them & prefer to use more powder instead. The double primers required a reduction of 8gr. of powder for identical results - in my rifle, with those loads they were designed for.

A problem with straight sided cases is a poor 'hold' or grip on the bullet. The bullet enters the throat easily from the case, which rapidly increases the 'volume' of the space for the powder to manifest it's burn, which can cause a pressure drop with poorly ignited or hard to ignite powders. In worse case senarios, the result is as Dan described in hang-fires. And, as he stated, this is right on the edge of a disastrus blow-up. This is why straigth cases have difficulty igniting/burning the slower powders. This pressure drop, then pressure climb once the powder does start to burn properly, can produce wave action of pressure, which is one of the perceived causes of detonations.

The grip on the bullet can be increased by reducing the diameter of the case neck expander button, which will produce a tighter hold on the bullet. Couple a tighter hold on the bullet, with a good heavy crimp in a proper crimping groove or the use of a Lee crimper to impart or impress a crimping groove in a straight sided bullet helps with strong igntion and better results occur.