i've got this from another forum about using Dacron. i use the hardwood floor.
Dacron is long fibered and springy, and can hold powder in place that weighs considerably more than the dacron, but you must give it some help by:
1) Ensuring that the dacron fills the entirety of the available space, and
2) is of sufficient density to fill that space with enough resistant dacron that the powder cannot shift around.
I suggest inserting some dacron in a charged case, then removing it and comparing its volume to that of the space you want to fill. What you see is similar to its condition at repose in the case.
Further, once you add dacron, slam the case, mouth down, as hard as you can manage against the top of your loading bench. Toss it against the carpeted floor, then, to ease your mind, against a hardwood floor if you're the inquisitive sort.
See how this holds up and if it fully prevents the powder from position shifts.
It should be able to resist some pretty serious G's in terms of impact and ability to hold the powder in place.
Depending upon the size of the case and the space to be filled, amounts can range from a small fraction of a grain to a few grains or so.
It should not be "packed" in as in the sense of tamping it down hard. Just push it in the case until resistance is lightly felt, ensuring the available space is filled from powder to where the bullet's base will be. Ensure that the dacron, before seating the bullet, will be a bit above the final position of the bullet's base.
Now test it. Visibly, the dacron should be substantial enough, when viewed from the top, that the powder cannot be seen through it. It should look "filled in" but not solid. Start the inertia testing after this is verified.
After the inertia testing, carefully remove the dacron and examine it. There should be no substantial amount of powder contained within the dacron fibers, and powder should not come out of the case along with the dacron when it is removed. The dacron should still be resting above the powder as it is withdrawn with no intermixing of the two after the inertia tests.