I'll put in my 2 cents here. First, on that 600 JDJ I would definately not use fillers with a powder as fast as WSL. Any powder with as much energy as that has does not respond well to a compressed load. Another thing to keep in mind is there two kinds of fillers. One is a tuft of kapok or dacron on top of the powder, and another is a case full of cream 'o' wheat or shot buffer. With the kapok you run the risk of creating a secondary projectile, having it hit the base of the bullet, and ring the chamber. With the solid fillers this doesn't happen, but if you use them in a bottleneck case you can send pressures sky high as it is all trying to make it through the little hole. The main thing to guide you is the behavior of the powder. If you, like me, like to use a lot of mil surp powder in applications that it's not designed for, fillers can occasionally be useful. Two noticable uses I routinely do are with WC680. This stuff is nearly the same as AA1680, and it will not tolerate less than a full case. It is very erratic at low pressures. I use it in my 7.62x39 cast bullet rifle, and in the 45-70. In both cases I do not want a full case load, because the velocity is too high. So I use Ballistic Products shot buffer to take up the space. The loads are super accurate, extremely consistant, and burn clean. The filler boosts pressures enough to get that powder to work right. But something like WSL or any other fast burning pistol/shotgun powder should never be used with fillers. It's easy to either ring your chamber or skyrocket pressures and hurt yourself with these, and since they light off so easily there's no need for fillers anyhow. Now probably the best thing to do in big cases to keep the speed down is to use powder that's slower than optimal such as 4350 in the 45-70. It can be loaded to full capacity, shoots good, and can't hurt you. Fillers can be very useful if you know what you're doing, but I never recommend their use to anyone who is not intimately familiar with the behavior of the powder they are using and knows exactly what is going to happen before they touch it off. In other words, most people should not think about messing with fillers!