I've been searching for info on using Dacron filler for downloaded cartridges, specifically 45/70. In the course of said surfing I found info on using cardboard tubes in a case to reduce the case volume while keeping a powder column that did not have an airspace in front of it between powder and bullet. This seems to relate mainly to black powder loads, in particular the reduced cavalry load for the 45/70. For anyone that does not know, an airspace between black powder and the bullet is considered to be quite dangerous, at least to the firearm and possibly to the shooter.
I'm really just sharing this for information and comments. It seems to make sense as far as black powder is concerned, and maybe some reduced load nitro.
By way of disclaimer, I have no idea whatever of the effectiveness of this approach, neither any dangers, although I doubt there would be any, but that's just my opinion. which lack of knowledge rarely stops me from offering.