Re: 308 Sub Sonic case filler
When I load subs, my gun likes around 10.3ish grains. Wind up around 1040 or so fps.
I have decent case fill.
A couple things you can do: Run as heavy a bullet your barrel will take. Seat your round fairly deep; dont chase the lands. Doing things like this and tweaking the load have gotten me consistent 1 minute loads. Usually under that, but hardly ever worse. I use mag primers as well. They seem to help with consistency, and tighten up the groups a bit YMMV.
I have heard of cotton being used for a filler, but I have not tried it. May have to some time, but I have been pretty happy how my subs have been running.
I assume the cotton would be totally gone before the round ever got down the barrel. Any other ideas on it?
Have you worked any sub loads for your rifle yet? I wager you will be 10.5ish grains, not 9. That bit makes a difference as TB is quite "fluffy."