A friend suggested the use of amaranth seeds for filler in BP loads.   It works great!   It is a hard spherical grain about 1mm in diameter, and meters nicely in a powder measure.  Just dump in however much on top of the powder and compress however much you want.  Give it a try-- costs only ca $3/lb.

CHRIS
RGChristensen

   Will amaranth seeds be abrasive?   I wouldn't think so.  Since it is sold as a food product (I get it from a health food store) it is probably pretty clean.   I suppose that a fellow could ash some of it down and see how much residue there was, and what it looked like under a microscope -- looking for silica particles or whatever.
   It is probably less abrasive than paper patched bullets.

CHRIS
RGChristensen

If it is what I think it is you don't want that crap growing anywhere!!!

Rendering the seeds infertile could be done - heat.

Likely not abrasive - most seeds contain a fair amount of oil.

Where do you get it?

   We had joked, of course, about the ranges growing up to amaranth plants.  What I will do is pop off a round in the cellar tomorrow and catch some of the seeds.  If they are still viable, I will take TIM's suggestion and toast them to ca 200F and sterilize them.

   They are not very oily.   I had tried flax seeds, and they are oily enough to make quite a mess when you try to compress them.

CHRIS
RGChristensen

I'd be leery of shooting something hard as a filler in my barrels - after all, impact at 1000+ fps makes me think of something like shotblasting or sandblasting the bore... And as to uniform compression, why don't you just compress before adding the filler?  Problem solved, you can use a soft, compressible filler - if any is needed.  
IMO, the best filler is air - even cheaper, not abrasive, and no need to meter anything.  And yes, I do use it in BP loads, and no, it doesn't ring the barrel or blow up the rifle.  I do keep the case filled at least 75-80% though.
Another neat filler is BP...

  Since we're on the general topic of B.P. case fillers in general, and the criteria specifically of the best available fillers, perhaps an assessment of "Puff-Lon" may add to the body of knowledge, whether pro or con.  I've never used it personally, but would appreciate learning from those who have.  I do note the provider describes it as being "lubricating ballistics filler".

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C.M.M.
.

I tried puff-lon, hoping it would fix a particular leading problem in one of my rifles.  As far as I can tell, it had no effect, good or bad, on the accuracy, velocity, pressures, leading, or any other feature of the rifle or load.   It did add to the loading time, and is fairly irritating to get into the cases and clean up your loading blocks after use.  Too bad, from the advertisement, I thought it was some of the long lost magic of the ancients, like Leupold Wads.  From inspection, it appears to be a mixture of finely ground cellulose and graphite powder.

dave