I have 3 Custer range Trapdoor carbines and I want to make up a few close to original loads and shoot one of them. The one I want to shoot is a 4-digit and its in outstanding shape. I've loaded many thousands of rounds but not one BP round. A friend of mine cast me some 405's and I got a can of Goex from another friend. I have 20 pcs of new brass to make some rounds with. My main question is how to measure and add the powder to the cases. Can I meter out 55 grns, pour it in the case, measure where it comes up to inside the case, check that against the bullet depth and add a filler so the powder slightly compresses, or do I have to meter the 55grns, tap the case a certain number of times to set the powder and then then check the bullet depth against the spacer thickness? I've read that with BP, powder is measured by volume and not weight, what is the deal with 55grns of volume compared to 55grns of weight? Is the important thing to make sure there is no air space between the powder and the bullet? Thanks

Black powder is a  Volume measurement use a powder measure for a muzzleloader.
I have made a couple of 45-70 using Tripple7 and lead bullet.
No wadding that is used with round balls.

BP Cartridge reloading is a whole nuther ball game. I've always just measured my charges by weight, but I might look into using a volume measurer, just to see how it goes. If you use a powder hopper, you will want to get one designed for BP, just to err on the safe side. What ever you do, use BP lube on your bullets, make sure all the space is filled and without gaps and compress you powder at least some.

You will need to take up space, use wads and not some silly cereal. You can get wadding of various thicknesses through Buffalo Arms, Track of the Wolf and many others.

Black powder cartridge is a bit more work, but it's so worth it. Be careful, next thing you know you will be ordering a Shiloh to shoot in competitions.

1.5Fg or 2Fg will probably give you best results.

Jupiter Max has all the important points.  All I'm going to add is that while I use a volumetric measure I set that measure by weight.  BP density is much more inconsistent lot to lot than smokeless and I find I get more repeatable results lot to lot setting the measure with my powder scale rather than relying on some marks on the side of a BP powder measure.

I'm loading full charges of BP in the .50-70.  In order to get the powder in the case I use a 36" drop tube to get the powder in the case w/o overflowing the case.  Then I use a compression die, NOT THE BULLET, to compress the powder to a consistent depth in the case.  Then I seat the bullet.  A few thousands up to maybe 1/16" gap between the compressed powder and the base of the bullet is OK but I do aim for as small a gap as possible.  The reason you do not compress the powder w/ the bullet is your bullet should be quite soft for BP and you will deform the bullet if you use it to compress the powder.  I use a 25:1 lead tin mix.

55 Grains of Black Powder is 55 Grains however This is not true of the substitutes. 55 Grains of BP will fit in a 45-70 case easily and may require a small amount of filler.  The Lee 405 HB bullet is supposed to be a near copy of the original bullet.

Charging cases for BP loads need not be complex. Remember that the old timers used a simple scoop measure (poured through a drop tube if possible). You can easily make a scoop out of an old cartridge case with a simple wire handle.

Lee makes a pretty good copy of the old 405-grain carbine bullet. It is hollow base, and if cast soft, will slug up to fit the old trap door barrels which are often pretty large in groove diameter.

I’d load 62 grains by volume and a card wad over. Should allow  the bullet to just sit on the powder. Remember , you don’t want any space between the powder and the bullet or you will make a pipe bomb

If you don't use a drop tube I find that flicking the base of the case while holding near the mouth a few times will settle the powder.