Daryl,
Accuracy is good with 70 and 90 in my 1886.
The 70 bullet enters the barrel before it leaves the brass 70 case, so it is always guided/supported. Similar to a .38 fired in a .357 Magnum revolver. I have done that for decades.

I have never observed any copper or lead left by the passage of the 70 bullet from case to barrel.

I never let the chamber or barrel become fouled before cleaning it. Better to avoid such problems than to pay to fix them. Another nice thing about the TD configuration - easy to get to the chamber and rifling for cleaning.
Tomorrow I will take my 1886 down to insert each newly loaded 90 round into the chamber just to be sure there will be no issues when I do get out with my chrony to shoot all 4 different loadings and get velocity readings.

I just got the go ahead today from my smith on extending the chamber of my 45-70 to 45-90 length and regulating with my 90 hand loads. Good fun! Interesting that both the 1886 and 45-70 DR have 26 inch barrels.

A comment on accuracy.
The barrels of all my rifles are capable of sub minute of angle accuracy. All my .458 barrels have placed 2-3 sequential shots in the same hole in a target. That is as far as I go to measure hunting accuracy because the guns are more accurate than I. In the field, we just do the best we can under the circumstances and that is usually good enough.

Cheers and sleep well. I will as it is now raining and that usually makes for a good nights sleep.