Chambering may be difficult if you are compressing without a compression die because when you are compressing the powder without a compression die you are compressing the powder with the bullet & deforming the bullet
I think you mean that you are engraving .008 not .8 or 8/10
I would try .002-.004 for the engraving but each gun has it's own mind.
My 45-70 shoots great seated .005 off of the lands & chambers like a charm.
Starting loads & load development, see below.
http://www.shootersforum.com/blackp....com/blackpowder-cartridge-shooting-loading/56122-45-70-black-powder-loads.html
http://www.shootersforum.com/blackp...um.com/blackpowder-cartridge-shooting-loading/45174-how-much-black-45-70-a.html
Your overall length should be the same, at least for all rounds in the same test.
When you increase or decrease your OAL you also need to reset your compression die.
Accuracy requires consistancy .
It would help if you fireformed all your brass with smokeless such as Accurate 5744 then necksizing only before working up a BP load,
This would give you the same case volume as brass neck sized after fireing & also get you some trigger time with your new rifle.
Shoot some groups & let the barrel cool between groups. If you shoot too much too fast you will get a lead wash in your barrel that is a PITA to clean.
If you have or can get some Lyman 457193 27 grains of accurate 5744 shoots great.
For the heavier bullet that you have play around with 23-28 gr of 5744
These are Springfield Trapdoor level loads & are lighter than 45-90 loads