Interesting discussion. Tagged.

If there isn't you're likely to get a BIG surprise. :D

And this is a reason I asked about powder compression. Reading the boards some guys complain about using powders that require them to damn near jam the bullet in to get the desired OAL. So, as I'm reading this, I'm thinking about the PSI of packed powder. I'm sure this all has to do with burn rates in some way to max out the load.

I keep chambers loaded with the guns in rotation. Not having to unload sd round/s to shoot to practice, having dedicated practice guns. No need to worry about set back from constant chambering. I worry more about mucking the rim up so the extractor doesn't get a solid a purchase as it should on a new round. Making sure to load the round back in the mag [ to rechamber it ] with a clean spot on the rim, not on a gouged/deformed areas previously grabbed by the extractor :)

I get that, Brownie....but why are fillers used when reloading?

In the case of position sensitive powders, to keep the powder in proximity to the primer to ensure uniform ignition, which it would not achieve if laying on one side of the case

So some sort of compression or uniformity of the powder is necessary?

Remember you are talking about 3 different variables here.

1 Powder
2 Bullet
3 OAL

Not all 150 ge bullets are the same length. Not all powders have data for all calibers and OAL is dependent upon the bullet length and the powder charge. That is why different powders have different OAL for one bullet and may have a different OAL for the same bullet with a different powder charge.

^this

And I've never heard of fillers used in pistol reloading

No compression with fillers, but with some loading real powder puffs, depending on powder used, it can be so little compared to case capacity ignition is spotty and I've been told as dangerous as a double charge.