Back in the day, did ammunition manufacturers ever use fillers in black powder cartridges to reduce the powder charge for a lighter load? Or were they always full rock & roll?
I don't know how I missed this posting way back when, but I will try to give you some form of definitive info.
What most have stated about downloading of factory casing is totally correct. When a company developed a 'new cartridge' it was for a reason and they produced them as developed. But as someone stated above the 'reduced 45 Colt' load was one of the best examples of manufacturing a known round for a specifically different market.
But to change your question into a different direction than the primary manufactures, I will point out this little know piece of information. This come out of the book 'Sharps Rifle -- The Gun that Shaped American Destiny' by Martin Rywell. In this book are several Sharps Company ads or booklet write-ups. The one I am referring you to is on page 111, under the title 'Reloading Implements, Sight, etc. -- Long Range Reloading Implements'. The last entry on this list of available purchase items -- is "
Corks use in loading shells, when a charge of powder less than the capacity of the shell is required, per 100.
As you can see the manufactures did know that not everyone always wanted to fire 'full loads' in the only firearm they had 'all the time'. So they did accommodate for that factor. Yes, these are not as you posted factory load but 'Your Self-Loading Desires'.
Hope this helps your questioning.
BRS