This problem reared it's head in some of the largest sporting rifle rounds ever shot with the advent of modern IMR type smokeless powders.
Many of the big double rifle rounds were originally for black powder and all of them quickly went to cordite when that became available.
Cordite was the perfect answer physically because of it's spaghetti like nature.
While not filling the entire volume, it did stay still occupying the entire length of the brass.
Some even had the necks formed after the cordite was inserted.
Cordite's downside was great temperature sensitivity.
Rifles started to blow after being sent to India and especially Africa.
The loads were reduced for tropical use.
Cordite eventually went away and I am not sure how much was ever available to handloaders.
Along comes the DuPont type powders we are familiar with.
They pack into a much smaller space for a given weight than cordite.
Double rifle reloaders quickly tried every filler imaginable and the factories (Eley and Kynoch) settled on a foam plug very like foam ear plugs.
They are still used in factory DR ammo and are available from Kynamco.
Their price precludes me from using them recreationally in target 44 ammo.
I settled on Grex shotshell buffer.
It's been out of print for quite awhile but Ballistic Products makes similar buffers.
The trick with these light powdered plastics is to make sure everything is under compression so the contents don't mix or move.
A typical load I'd make again because it's accurate:
44 Special, WW brass, WLP primer, 4.75 gr PB, 10 gr Grex, 240 SWC.
These chrono from 675 to 700 fps out of various 3" revolvers with low variance and excellent targets.
I
think the filler also helps bump up or obturate the bullet by providing a quick spike in pressure as the plastic vaporizes.
It may help lubricate the bullet's ride as well (or the next one).
One of my best filled loads. Too bad PB is out of print now too.
Tip: Don't use any paper product for filler if you intend on using an indoor range.
You will leave an unholy mess on the floor in front of you.
Don't ask how I know this.
