93X, If filler is properly used, no co-mingleing will occur, regardless of the type of filler used! The shifting of filler needs a place for the filler to shift to. A properly loaded NE round with filler, will not shift under any cercumstances.

The granulated fillers like shot buffers, are the worse offender to shifting, because the bullets are not seated down on top of the filler tight enough. No airspace is allowed in a case when any type of filler is used, this is why it is called "FILLER"! Once the charge is dumped into the case the filler has to be enough to fill the case enough so that when the bullet is seated, the filler is slightly compressed. Most of the ringed chambers are caused by useing filler like Creame of wheat, which should NEVER be used as filler. People got the ideaof useing C.O.W. from old books where it was used for fireforming wildcat cases. This was done without a bullet in the case. The case was loaded with a small amount of a fast burning powder, on the primer, then the case filled with C.O.W., no bullet, and with the rifle held barrel pointed at the sky, fired. This fireformed case very well, but would have been disasterous if a bullet had been seated as well. The other ringing is caused by idiots loading very light loads in a large case, and seating a wad down on top of the powder, then seating a bullet, leaving a large amount of airspace between the wad, and the bullet. When this is fired, with the wad seated that way, the wad becomes the projectile, and a short distance away the bullet becomes a secondary projectile, obstruction, much the same as another bullet in the barrels, ahead of the one being fired.

The two best fillers are the dacron fiber, and the foam plugs, but both are to be used to absolutely fill the space between the powder charge and the bullet, and be slightly compressed by the bullet being seated to the proper depth. "NO AIRSPACE"!!!!!!
There is no need for ultra light loads in any firearm. Light loads have destroyed far more firearms that over loads!

Handloading is a must with a double rifle for two reasons, #1 is because that is the only way you will get the best from your rifle, and sencondly, because of costs that most of us must take into consideration. When my life may be on the line, I trust my handloads far more that any factory worker, who may be coming off a week long drunk, when he loads my ammo.